Checkbox

[pela]

prova così:

<script language="javascript">
function controlla(){
  var MAXSEL=2;
  var count=0;
  var cc=document.myform.mycheck;
  for(i=0;i<cc.length;i++)
	if(cc[i].checked) count++;
  if(count>MAXSEL){
    alert("Puoi selezionare al massimo "+MAXSEL+" checkbox");
    return false;
  }
  return true;
}
</script>

<form name="myform">
<input type="checkbox" name="mycheck" onclick="return controlla()"> CHECK1 

<input type="checkbox" name="mycheck" onclick="return controlla()"> CHECK2 

<input type="checkbox" name="mycheck" onclick="return controlla()"> CHECK3 

<input type="checkbox" name="mycheck" onclick="return controlla()"> CHECK4 

</form>