var str = 'Some very long string';
if(str.length > 10) str = str.substring(0,10);
function limit(element)
{
var max_chars = 2;
if(element.value.length > max_chars) {
element.value = element.value.substr(0, max_chars);
}
}
<!-- HTML -->
<input type="number" onkeydown="limit(this);" onkeyup="limit(this);">