$chkcount = 0;
  if (isset($_POST['check1'])) && ($_POST['check1']){ ++$chkcount;}
  if (isset($_POST['check2'])) && ($_POST['check2']){ ++$chkcount;}
....
  if (isset($_POST['check9'])) && ($_POST['check9']){ ++$chkcount;}
if ($chkcount > 1){
    // multiple checkboxes checked
   } else {
  // one or zero checked
 }