<div id="rein"></div>
<script>
  d1=document.getElementById('rein');
for(p=0;p<=20;p++){
var but = document.createElement("input");
  but.class="buttonclass";
  but.id='button'+p+'';
  but.type='button';
  but.value='Button '+p;
  but.addEventListener("click", function(){
   alert(this.id)
}); 
  d1.appendChild(but);
}
</script>