<script type="text/javascript">
function checkboxlimit(checkgroup, limit){
for (var i=0; i<checkgroup.length; i++){
checkgroup[i].onclick=function(){
var checkedcount=0
for (var i=0; i<checkgroup.length; i++)
checkedcount+=(checkgroup[i].checked)? 1 : 0
if (checkedcount>limit){
alert("You can select a maximum of "+limit+" boxes.")
this.checked=false
}
}
}
}
checkboxlimit(document.forms.name-of-your-form['field-name[]'], 5);
</script>