JSFiddle - React, Tailwind, and code Playground

HTML

<div id="cat-0">
  <div class="item"></div>
  <div class="item"></div>
</div>
<div id="cat-1">
  <div class="item"></div>
  <div class="item"></div>
  <div class="item"></div>
</div>

JavaScript

$('[id^="cat-"]').each(function(){
    var items = $('.item', this);
    alert(this.id+': '+items.length); // => 2, 3
});