This might help..
<script type="text/javascript">
function create_textbox(container, count){
var cont = document.getElementById(container);
// reset container contents
cont.innerHTML = "";
for(var i=0; i<count; i++){
var input = document.createElement("input");
input.type = "text";
input.name = "name"+i;
cont.appendChild(input);
}
}
</script>
<form method="POST" action="<?php echo $_SERVER['PHP_SELF']; ?>">
<select name="textboxes" onchange="create_textbox('input_container', this.value)">
<option value="1">1</option>
<option value="2">2</option>
<option value="3">3</option>
<option value="4">4</option>
<option value="5">5</option>
<option value="6">6</option>
<option value="7">7</option>
<option value="8">8</option>
<option value="9">9</option>
<option value="10">10</option>
</select>
<div id="input_container"></div>
</form>