JSFiddle - React, Tailwind, and code Playground

HTML

<ul class="relatedelements">
   <li style="display:none;" class="1">anything</li>
   <li style="display:none;" class="2">anything</li>
   <li style="display:none;" class="3">anything</li>
</ul>
<div class="num-relatedelements"></div>

JavaScript

$(document).ready(function() {
  $('.2').show();

  var numrelated=$('.relatedelements > li:visible').length;
  $('.num-relatedelements').html(numrelated);
});