JSFiddle - React, Tailwind, and code Playground

HTML

<select size="5" multiple="multiple" id="selectMultiple">
    <option id="2">Comedy</option>
    <option id="3">Funny</option>
    <option id="4">Love</option>
</select>
<div id="howMany"></div>

JavaScript

$("#selectMultiple").change(function() {
    var howMany = $("#selectMultiple option:selected").length;
    $("#howMany").text( howMany );
})