Перенос виджетов при изменении размеров окна

Вот вам простенький вариант того что вы хотите. Я поэкспериментировал с "Окно 1" и "Окно 2".

Обратите внимание, что изменил ОДНУ строку в вашей форме. Пробуйте.

from PyQt5 import QtCore, QtGui, QtWidgets
from PyQt5.Qt import *


class Ui_MainWindow(object):
    def setupUi(self, MainWindow):
        self.centralwidget = QtWidgets.QWidget(MainWindow)
        
#        self.horizontalLayout_16 = QtWidgets.QHBoxLayout(self.centralwidget)      # ---
        self.horizontalLayout_16 = QtWidgets.QHBoxLayout()                         # +++
        
        self.scrollArea_2 = QtWidgets.QScrollArea(self.centralwidget)
        self.scrollAreaWidgetContents_9 = QtWidgets.QWidget()
        self.scrollAreaWidgetContents_9.setGeometry(QtCore.QRect(0, 0, 76, 200))
        self.horizontalLayout_9 = QtWidgets.QHBoxLayout(self.scrollAreaWidgetContents_9)
        self.label_2 = QtWidgets.QLabel(self.scrollAreaWidgetContents_9)
        self.horizontalLayout_9.addWidget(self.label_2)
        self.scrollArea_2.setWidget(self.scrollAreaWidgetContents_9)
        self.horizontalLayout_16.addWidget(self.scrollArea_2)
        self.scrollArea = QtWidgets.QScrollArea(self.centralwidget)
        self.scrollAreaWidgetContents_8 = QtWidgets.QWidget()
        self.scrollAreaWidgetContents_8.setGeometry(QtCore.QRect(0, 0, 76, 200))
        self.horizontalLayout_8 = QtWidgets.QHBoxLayout(self.scrollAreaWidgetContents_8)
        self.label_6 = QtWidgets.QLabel(self.scrollAreaWidgetContents_8)
        self.horizontalLayout_8.addWidget(self.label_6)
        self.scrollArea.setWidget(self.scrollAreaWidgetContents_8)
        self.horizontalLayout_16.addWidget(self.scrollArea)
        self.scrollArea_3 = QtWidgets.QScrollArea(self.centralwidget)
        self.scrollAreaWidgetContents_10 = QtWidgets.QWidget()
        self.scrollAreaWidgetContents_10.setGeometry(QtCore.QRect(0, 0, 76, 200))
        self.horizontalLayout_10 = QtWidgets.QHBoxLayout(self.scrollAreaWidgetContents_10)
        self.label_9 = QtWidgets.QLabel(self.scrollAreaWidgetContents_10)
        self.horizontalLayout_10.addWidget(self.label_9)
        self.scrollArea_3.setWidget(self.scrollAreaWidgetContents_10)
        self.horizontalLayout_16.addWidget(self.scrollArea_3)
        self.scrollArea_4 = QtWidgets.QScrollArea(self.centralwidget)
        self.scrollAreaWidgetContents_11 = QtWidgets.QWidget()
        self.scrollAreaWidgetContents_11.setGeometry(QtCore.QRect(0, 0, 77, 200))
        self.horizontalLayout_11 = QtWidgets.QHBoxLayout(self.scrollAreaWidgetContents_11)
        self.label_14 = QtWidgets.QLabel(self.scrollAreaWidgetContents_11)
        self.horizontalLayout_11.addWidget(self.label_14)
        self.scrollArea_4.setWidget(self.scrollAreaWidgetContents_11)
        self.horizontalLayout_16.addWidget(self.scrollArea_4)
        self.scrollArea_7 = QtWidgets.QScrollArea(self.centralwidget)
        self.scrollAreaWidgetContents_14 = QtWidgets.QWidget()
        self.scrollAreaWidgetContents_14.setGeometry(QtCore.QRect(0, 0, 76, 200))
        self.horizontalLayout_12 = QtWidgets.QHBoxLayout(self.scrollAreaWidgetContents_14)
        self.label_15 = QtWidgets.QLabel(self.scrollAreaWidgetContents_14)
        self.horizontalLayout_12.addWidget(self.label_15)
        self.scrollArea_7.setWidget(self.scrollAreaWidgetContents_14)
        self.horizontalLayout_16.addWidget(self.scrollArea_7)
        self.scrollArea_8 = QtWidgets.QScrollArea(self.centralwidget)
        self.scrollAreaWidgetContents_15 = QtWidgets.QWidget()
        self.scrollAreaWidgetContents_15.setGeometry(QtCore.QRect(0, 0, 76, 200))
        self.horizontalLayout_13 = QtWidgets.QHBoxLayout(self.scrollAreaWidgetContents_15)
        self.label_16 = QtWidgets.QLabel(self.scrollAreaWidgetContents_15)
        self.horizontalLayout_13.addWidget(self.label_16)
        self.scrollArea_8.setWidget(self.scrollAreaWidgetContents_15)
        self.horizontalLayout_16.addWidget(self.scrollArea_8)
        self.scrollArea_6 = QtWidgets.QScrollArea(self.centralwidget)
        self.scrollAreaWidgetContents_13 = QtWidgets.QWidget()
        self.scrollAreaWidgetContents_13.setGeometry(QtCore.QRect(0, 0, 76, 200))
        self.horizontalLayout_14 = QtWidgets.QHBoxLayout(self.scrollAreaWidgetContents_13)
        self.label_17 = QtWidgets.QLabel(self.scrollAreaWidgetContents_13)
        self.horizontalLayout_14.addWidget(self.label_17)
        self.scrollArea_6.setWidget(self.scrollAreaWidgetContents_13)
        self.horizontalLayout_16.addWidget(self.scrollArea_6)
        self.scrollArea_5 = QtWidgets.QScrollArea(self.centralwidget)
        self.scrollAreaWidgetContents_12 = QtWidgets.QWidget()
        self.scrollAreaWidgetContents_12.setGeometry(QtCore.QRect(0, 0, 76, 200))
        self.horizontalLayout_15 = QtWidgets.QHBoxLayout(self.scrollAreaWidgetContents_12)
        self.label_18 = QtWidgets.QLabel(self.scrollAreaWidgetContents_12)
        self.horizontalLayout_15.addWidget(self.label_18)
        self.scrollArea_5.setWidget(self.scrollAreaWidgetContents_12)
        self.horizontalLayout_16.addWidget(self.scrollArea_5)
        MainWindow.setCentralWidget(self.centralwidget)

        self.retranslateUi(MainWindow)
        QtCore.QMetaObject.connectSlotsByName(MainWindow)

    def retranslateUi(self, MainWindow):
        _translate = QtCore.QCoreApplication.translate
        self.label_2.setText(_translate("MainWindow", "Окно 1"))
        self.label_6.setText(_translate("MainWindow", "Окно 2"))
        self.label_9.setText(_translate("MainWindow", "Окно 3"))
        self.label_14.setText(_translate("MainWindow", "Окно 4"))
        self.label_15.setText(_translate("MainWindow", "Окно 5"))
        self.label_16.setText(_translate("MainWindow", "Окно 6"))
        self.label_17.setText(_translate("MainWindow", "Окно 7"))
        self.label_18.setText(_translate("MainWindow", "Окно 8"))


class ListWidget(QListWidget):                                                   # +++
    def __init__(self, *args, **kwargs):
        super(ListWidget, self).__init__(*args, **kwargs)
        self.setHorizontalScrollBarPolicy(Qt.ScrollBarAlwaysOff)
        self.setEditTriggers(self.NoEditTriggers)
        self.setFlow(self.LeftToRight)
        self.setWrapping(True)
        self.setResizeMode(self.Adjust)
        self.setSpacing(10)
        # Резиновая лента 
        self._rubberPos  = None
        self._rubberBand = QRubberBand(QRubberBand.Rectangle, self)

    def makeItem(self, lb):
        item = QtWidgets.QListWidgetItem(self)
        item.setSizeHint(QtCore.QSize(170, 120))          # попробуйте другой size
        self.setItemWidget(item, lb)


class Window(QtWidgets.QMainWindow, Ui_MainWindow):
    def __init__(self, parent=None):
        super().__init__()
        self.setupUi(self)
        
# +++ vvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvv        
        # Окно 1
        self.scrollAreaWidgetContents_9.resize(200, 200)
        # Окно 2
        self.label_6.setAlignment(Qt.AlignCenter)
        self.scrollAreaWidgetContents_8.resize(300, 300)
        self.horizontalLayout_8.setContentsMargins(0, 0, 0, 0)
        
        _listWidget = [
            self.scrollArea_2, self.scrollArea, self.scrollArea_3, self.scrollArea_4,
            self.scrollArea_7, self.scrollArea_8, self.scrollArea_6, self.scrollArea_5,
        ]

        self.listWidget = ListWidget()

        box = QtWidgets.QVBoxLayout(self.centralwidget)
        box.addWidget(self.listWidget)
                                              
        for lbl in _listWidget:
            self.listWidget.makeItem(lbl)        
# +++ ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^


if __name__ == '__main__':
    import sys

    app = QtWidgets.QApplication(sys.argv)
    app.setStyle('Fusion')
    app.setStyleSheet('QLabel {background-color: rgb(244,164,96);}')
    w = Window()
    w.resize(650, 450)
    w.show()
    sys.exit(app.exec_())

Такое можно сделать через FlowLayout, его нет в библиотеке, поэтому ниже реализация с примером.

Попробуйте:

from PyQt5.QtWidgets import QLayout, QSizePolicy
from PyQt5.QtCore import Qt, QRect, QSize, QPoint


# SOURCE: https://github.com/baoboa/pyqt5/blob/25bdb92c38d9c0a915c6366769cc00a63a1f04b2/examples/layouts/flowlayout.py#L65
class FlowLayout(QLayout):
    def __init__(self, parent=None, margin=0, spacing=-1):
        super().__init__(parent)

        if parent is not None:
            self.setContentsMargins(margin, margin, margin, margin)

        self.setSpacing(spacing)

        self.itemList = []

    def __del__(self):
        item = self.takeAt(0)
        while item:
            item = self.takeAt(0)

    def addItem(self, item):
        self.itemList.append(item)

    def count(self):
        return len(self.itemList)

    def itemAt(self, index):
        if 0 <= index < len(self.itemList):
            return self.itemList[index]

        return None

    def takeAt(self, index):
        if 0 <= index < len(self.itemList):
            return self.itemList.pop(index)

        return None

    def expandingDirections(self):
        return Qt.Orientations(Qt.Orientation(0))

    def hasHeightForWidth(self):
        return True

    def heightForWidth(self, width):
        height = self.doLayout(QRect(0, 0, width, 0), True)
        return height

    def setGeometry(self, rect):
        super().setGeometry(rect)
        self.doLayout(rect, False)

    def sizeHint(self):
        return self.minimumSize()

    def minimumSize(self):
        size = QSize()

        for item in self.itemList:
            size = size.expandedTo(item.minimumSize())

        margin, _, _, _ = self.getContentsMargins()

        size += QSize(2 * margin, 2 * margin)
        return size

    def doLayout(self, rect, testOnly):
        x = rect.x()
        y = rect.y()
        lineHeight = 0

        for item in self.itemList:
            wid = item.widget()
            spaceX = self.spacing() + wid.style().layoutSpacing(QSizePolicy.PushButton, QSizePolicy.PushButton, Qt.Horizontal)
            spaceY = self.spacing() + wid.style().layoutSpacing(QSizePolicy.PushButton, QSizePolicy.PushButton, Qt.Vertical)
            nextX = x + item.sizeHint().width() + spaceX
            if nextX - spaceX > rect.right() and lineHeight > 0:
                x = rect.x()
                y = y + lineHeight + spaceY
                nextX = x + item.sizeHint().width() + spaceX
                lineHeight = 0

            if not testOnly:
                item.setGeometry(QRect(QPoint(x, y), item.sizeHint()))

            x = nextX
            lineHeight = max(lineHeight, item.sizeHint().height())

        return y + lineHeight - rect.y()


if __name__ == '__main__':
    import sys

    from PyQt5.QtWidgets import QWidget, QPushButton, QApplication

    class Window(QWidget):
        def __init__(self):
            super().__init__()

            flowLayout = FlowLayout()
            flowLayout.addWidget(QPushButton("Short"))
            flowLayout.addWidget(QPushButton("Longer"))
            flowLayout.addWidget(QPushButton("Different text"))
            flowLayout.addWidget(QPushButton("More text"))
            flowLayout.addWidget(QPushButton("Even longer button text"))
            self.setLayout(flowLayout)

            self.setWindowTitle("Flow Layout")

    app = QApplication(sys.argv)
    mw = Window()
    mw.show()
    sys.exit(app.exec_())