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>

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