var counter = 1;
var limit = 99;
function addInput(divName){
     if (counter == limit)  {
          alert("You have reached the limit of adding " + counter + " inputs");
     }
     else {
          var newdiv = document.createElement('div');
          newdiv.innerHTML = "<label for='model_name'>Model" + (counter + 1) + "</label><input type='text' name='model_name[]' /><br /><label for='serial_number'>Serial Number</label><input type='text' name='serial_number[]' size='30' /><br />";
          document.getElementById(divName).appendChild(newdiv);
          counter++;
     }
}