I think that's the maximum for a single dialog. You could work around it by having something like a master dialog with a drop down box, then different dialogs that are activated based upon the selection in the drop down box, and have a command in each of those dialogs similar to
ON *:DIALOG:dialog_name:CLOSE:*:{
dialog $iif(!$dialog(master_dialog),-md,-ev) master_dialog master_dialog
}
That way when the sub-dialog (for lack of a better term) closes, the master dialog will automatically re-open.