Получить позицию курсора и проверить количество символов в contenteditable

$(document).ready(function() {
  document.execCommand('defaultParagraphSeparator', false, "p");

});

.svg {
  display: none;
}

.editor-box {
  position: relative;
}

.editor-box__title {
  position: relative;
  background: transparent;
  display: block;
  width: 100%;
  font-size: 38px;
  margin: 0 0 36px;
  line-height: 1.11;
  overflow: hidden;
  text-overflow: ellipsis;
  box-shadow: none;
  font-weight: 600;
}
.editor-content {
  position: relative;
  z-index: 1;
  min-height: 200px;
  padding: 20px;
  border: 1px solid #000;
}

<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>

<div class="editor-box">
  <div class="editor-content" id="editor-content" contentEditable="true">
    <br>
  </div>
</div>