MC-XOBAHCK,
<!DOCTYPE html>
<html>
<head>
<title>Untitled</title>
<meta charset="utf-8">
</head>
<body>
<div class="wrapTest">
<div class="test">
<input type="text"> <span class="unit">м</span>
<input type="text"> <span class="unit">м</span>
<input type="text"> <span class="unit">м</span>
<input type="text"> <span class="unit">м</span>
</div>
<div class="test">...</div>
<div class="test">...</div>
</div>
<p></p>
<script>
document.querySelector('.wrapTest').addEventListener('focus', function(e) {
let el = e.target;
if (el.tagName == 'INPUT') {
let inputs = el.parentNode.querySelectorAll('input');
let index = [].indexOf.call(inputs, el);
document.querySelector('p').innerHTML = index;
}
}, true);
</script>
</body>
</html>