If you are seated together, yes, the mandatory service charge is assessed (and yes, despite the standard disclaimer, it's possible the restaurant will be able to combine tables to accommodate your entire party).
If you are seated separately but at two tables of six, ditto.
If you are seated at three or more separate tables (so, fewer than six diners at each table) but all have the same server, yes, you will be assessed the 18% service charge.
If you are seated at three or more separate tables and each table has a different server, no, you get to determine the tip percentage yourselves.