code: function( tmp, self ) {
        var cont = $( self.form.querySelectorAll('input[type=checkbox][name="'+ tmp.el.name +'"]') );
        // we dont want to open an error window for all checkboxes which have same "name"
        if ( cont.index( tmp.el ) !== 0 ) return;
        var count =  cont.filter(':checked').length;
        if ( count === 0 ) return;
        return count <= tmp.arg || messages.maxChecked.replace( '{count}', tmp.arg );
      }
docs: Checkbox check