Using CSS Counter, counter-reset, and counter-increment.

<h1>Testing the Counter</h1>
<ul>
  <li>ITEM 1</li>
  <li class="invalid">ITEM 2 - is invalid</li>
  <li>ITEM 3</li>
    <li class="invalid">ITEM 4 - is invalid</li>
    <li class="invalid">ITEM 5 - is invalid</li>
    <li class="invalid">ITEM 6 - is invalid</li>
</ul>
<p>DETAILS: </p>

<style>
body {
  counter-reset: invalidCount;
}

.invalid {
  background-color: pink;
  counter-increment: invalidCount;
}

p:after {
content: "You have " counter(invalidCount) " invalid entries";
}
</style>

EMBED:

See the Pen wvzpbma by rockykev (@rockykev) on CodePen.