JSFiddle - React, Tailwind, and code Playground

HTML

<div id="result-item-1" class="search-item " data-search-position="1">
    <div class='item-colours-result'>
        <ul class='list-class'><li>1</li><li>3</li><ul>
  </div>
</div>

<div id="result-item-2" class="search-item " data-search-position="2">
    <div class='item-colours-result'>
        <ul class='list-class'><li>1</li><li>3</li><li>5</li><ul>
  </div>
</div>

JavaScript

$('.search-item').each(function(index,item){
  var colorCount = $(item).find('.item-colours-result ul li').length;
  alert(item.id+':'+colorCount)
  $(item).attr('data-search-position',colorCount)
})