JSFiddle - React, Tailwind, and code Playground

HTML

<ul class="relatedelements">
   <li style="display:none;" class="1">anything1</li>
   <li style="display:none;" class="2">anything2</li>
   <li style="display:none;" class="3">anything3</li>
</ul>
<br />
Count: <span class="num-relatedelements">none</span>

JavaScript

$(function() {
    $('.2').show();
    var numrelated=$('.relatedelements > li:visible').length;
        $('.num-relatedelements').html(numrelated); 
    });