Here is an example that uses jQuery to achieve your goals:
Assume you have following html:
  <div>
    <select id="input_count">
      <option value="1">1 input</option>
      <option value="2">2 inputs</option>
      <option value="3">3 inputs</option>
    </select>
  <div>
  <div id="inputs"> </div>
And this is the js code for your task:
  $('#input_count').change(function() {
      var selectObj = $(this);
      var selectedOption = selectObj.find(":selected");
      var selectedValue = selectedOption.val();
      var targetDiv = $("#inputs");
      targetDiv.html("");
      for(var i = 0; i < selectedValue; i++) {
        targetDiv.append($("<input />"));
      }
  });
You can simplify this code as follows:
  $('#input_count').change(function() {
      var selectedValue = $(this).val();
      var targetDiv = $("#inputs").html("");
      for(var i = 0; i < selectedValue; i++) {
        targetDiv.append($("<input />"));
      }
  });
Here is a working fiddle example: http://jsfiddle.net/melih/VnRBm/
You can read more about jQuery: http://jquery.com/