Read the comments under the questions !
Also, creating proper elements is usually a good thing.
<div>
<input type="number" id="foo" value="3"/>
</div>
<div>
<script>
var i = document.getElementById("foo").value;
for (var num = 1; num <= i; num++) {
var elem = document.createElement('div');
elem.id = 'elem' + num;
elem.innerHTML = ' ... ';
document.body.appendChild(elem)
}
</script>
</div>