JSFiddle - React, Tailwind, and code Playground

HTML

<select name="extra_especialidad[]" multiple="multiple">
    <option selected value=1>One</option>
    <option selected value=2>Two</option>
    <option value=3>Three</option>
</select>

<select name="extra_especialidad[]" multiple="multiple">
    <option selected value=1>One</option>
    <option selected value=2>Two</option>
    <option value=3>Three</option>
</select>

JavaScript

var len = $("select[name='extra_especialidad[]'] option:selected").length;
alert(len);