impostare un numero massimo di checkbox selezionati

[br1]

<script>
var old=0;
function controlla(obj) {
f = obj.form;
rv = 0;
rv += (f.C1.checked)?1:0;
rv += (f.C2.checked)?1:0;
rv += (f.C3.checked)?1:0;
rv += (f.C4.checked)?1:0;
if (rv==1&&old==0) alert("ok");
if (rv>2) obj.checked=false;
old=rv;
}
</script>
<form name=mioForm>
<input type="checkbox" name="C1" value="ON" onclick="controlla(this)"> 
<input type="checkbox" name="C2" value="ON" onclick="controlla(this)"> 
<input type="checkbox" name="C3" value="ON" onclick="controlla(this)"> 
<input type="checkbox" name="C4" value="ON" onclick="controlla(this)"> 
</form> 

ciao