<div class="block">
  <div class="item"></div>
  <div class="item"></div>
  <div class="item"></div>
  <div class="item"></div>
  <div class="item"></div>
</div>

<div class="block">
  <div class="item"></div>
  <div class="item"></div>
  <div class="item"></div>
  <div class="item"></div>
  <div class="item"></div>
  <div class="item"></div>
  <div class="item"></div>
</div>

.block{
  display:flex;
}
.item{
  width: 50px;
  height: 50px;
  background:orange;
  margin:5px;
}

const blocks = document.getElementsByClassName('block');
const stats = [];
for(block of blocks) {
  const size = block.getElementsByClassName('item').length;
  stats.push(size);
}
alert(stats);