Here's an option using jQuery (jsFiddle)
HTML
Number of inputs:
<select class="numInputs" onChange="buildInputs(this);">
<option value="1">1</option>
<option value="2">2</option>
<option value="3">3</option>
<option value="4">4</option>
</select>
<div class="inputs"></div>
Javascript (jQuery)
$(document).ready(function () {
// set onchange event for the select box.
$('.numInputs').change(function () {
// clear any inputs added to the div.
$('.inputs').empty();
// get number of inputs to add from select box.
var num = $('.numInputs option:selected').text();
// build the inputs.
for (var i = 1; i <= num; i++) {
$('<input type="text"/><br/>').appendTo('.inputs');
}
})
});