How to add validation to limit characters in SharePoint Online Multiple line of text column

Mohammad Amer 

You can set the character limit by making use of maxlength attribute of TextArea element for Multiple lines of text column with "Plain Text". Inject the below script into NewForm.aspx and EditForm.aspx of SharePoint List and replace "FieldName" with your field name.

$( document ).ready(function() {
    $("textarea[id*=FieldName]").attr('maxlength','1000');
});

For Enhanced Rich Text: SharePoint renders this field as DIV with contenteditable attribute set. For this DIV, maxlength does not work. So need to control the keydown event to limit the characters in the DIV.

var max = 1000;
$("DIV[id*=FieldName][role=textbox]").keydown(function (e) { check_charcount(max, e); });
function check_charcount(max, e) {
if (e.which != 8 & e.which != 46 & e.which != 37 & e.which != 39
& $("DIV[id*=FieldName][role=textbox]").text().length > max) {
e.preventDefault();
}
}

Hope this helps!