Non lo trovi in giro, te lo faccio al volo:

codice:
<head>
...
<script type="text/javascript">
function addInputElement(ss){
  var i=0;
  var num = parseInt(ss.options.[ss.selectedIndex].value,10);
  for (i=1; i<=num; i++) {
    ss.form.elements['campo'+i].disabled = false;
  }
  for (; i<=6; i++) {
    ss.form.elements['campo'+i].disabled = true;
  }
}
</script>
<head>

<body>
<div>
  


    <select name="select1" onchange="addInputElement(this);">
      <option value="0">none</option>
      <option value="1">one</option>
      <option value="2">two</option>
      <option value="3">three</option>
    </select>
  </p>
  


    <form id="form1" name="form1">
      <input type="text" value="1" name="campo1" disabled="true">
      <input type="text" value="2" name="campo2" disabled="true">
      <input type="text" value="3" name="campo3" disabled="true">
      <input type="text" value="4" name="campo4" disabled="true">
      <input type="text" value="5" name="campo5" disabled="true">
      <input type="text" value="6" name="campo6" disabled="true">
    </form>
  </p>
</div>
</body>
Provalo: potrebbero esserci errori di sintassi.