[Триви]

Приблизительно так:

<style>
#udost, #udost_stat{
  display: none;
}
</style>

<input type="radio" name="doc_type" value="p" checked="checked"/> Паспорт
<input type="radio" name="doc_type" value="u"/> Уд. личности<br />
<input type="text" name="passport" id="passport" onkeyup="countDownchars(this.id, '7');" value="" maxlength=7> <span id="passport_stat">7</span>
<input type="text" name="udost" id="udost" onKeyUp="countDownchars(this.id, '9');" value="" maxlength=9> <span id="udost_stat">9</span>

<script type="text/javascript" src="http://code.jquery.com/jquery.min.js"></script>
<script type="text/javascript">
$('input[type=radio]').click(function(){
  if($(this).val()=='p') {
    $('#udost, #udost_stat').css('display', 'none');
    $('#passport, #passport_stat').css('display', 'inline');
  }
  if($(this).val()=='u') {
    $('#passport, #passport_stat').css('display', 'none');
    $('#udost, #udost_stat').css('display', 'inline');
  }
});

function countDownchars (element_id, maxchars) {
  $('#'+element_id).keyup(function (){
    number = $('#'+element_id).val().length;
    if(number <= maxchars){
      $('#'+element_id+'_stat').html(maxchars-number).css('color', '#000');
    }
    if(number == maxchars) {
      $('#'+element_id).attr({ maxlength: maxchars});
      $('#'+element_id+'_stat').css('color', '#f00');
    }
  });
}
</script>