Untitled

<!DOCTYPE HTML>
<html>
<head>
    <meta charset="utf-8">
    <title>dataset</title>

</head>
<body>
<input id="elem" data-length="5">
<script>
    let elem = document.querySelector('#elem');

    elem.addEventListener('blur', function(){
        if (elem.value.length == elem.dataset.length) {
            elem.value = elem.dataset.length;
        } else {
            alert('количество не совпадает')
        }
    });


</script>
</body>
</html>