You can get to the parent node and then query all the nodes with the class that is being searched. then we get the size
var parent = document.getElementById("parentId");_x000D_
var nodesSameClass = parent.getElementsByClassName("test");_x000D_
console.log(nodesSameClass.length);
_x000D_
<div id="parentId">_x000D_
<p class="prueba">hello word1</p>_x000D_
<p class="test">hello word2</p>_x000D_
<p class="test">hello word3</p>_x000D_
<p class="test">hello word4</p>_x000D_
</div>
_x000D_