intendi cosi:
codice:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
<title>Untitled Document</title>
<script type="text/javascript">
function contatore_checbox()
{
var conta=0
var limit=5, i, name;
var box = document.getElementsByTagName("input")
 for(i=1; i<box.length; i++)
  {
   if(box[i].checked && box[i].type == "checkbox")
    {
     conta++;
    }
 if(conta == limit)
  {
     box[i].checked = false;
	 conta--
    }
  }
}
</script>
</head>

<body>
<input name="group_1" id="group_1" type="checkbox" value="" />
<input name="group_2" id="group_2" type="checkbox" value="" />
<input name="group_3" id="group_3" type="checkbox" value="" />
<input name="group_4" id="group_4" type="checkbox" value="" />
<input name="group_5" id="group_5" type="checkbox" value="" />
<input name="group_6" id="group_6" type="checkbox" value="" />
<input name="group_7" id="group_7" type="checkbox" value="" />
<input name="conta" type="button" value="conferma" onclick="contatore_checbox()" />
</body>
</html>