Count number of li element and add class
Here $(this) is referring to box2 element.Only $('.box1 li').length is what you required
$('.box2').addClass(function() {
return 'list_' + $('.box1 li').length;
});.list_3 {
color: green;
}<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<ul class="box1">
<li>a</li>
<li>b</li>
<li>c</li>
</ul>
<div class="box2">text</div>Your query looks for .box1 li within .box2, though these two elements are siblings. Therefore, your find() query will always return 0.
For your query to work, your HTML would need to look like this:
<div class="box2">text
<ul class="box1">
<li>a</li>
<li>b</li>
<li>c</li>
</ul>
</div>
Without altering the structure of your HTML, you can get this to work by accessing .box1 li directly:
$('.box2').addClass(function(){
return 'list' + $('.box1 li').length;
});<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<ul class="box1">
<li>a</li>
<li>b</li>
<li>c</li>
</ul>
<div class="box2">text</div>