prova così:
codice:
<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>