I only have 4 on the system I'm replacing as that's all it can take - the two main front doors are serial'd on zone 1, the PIR in the passage by the keypad is zone 2 (with entry delay on it), the other downstairs rooms are serial'd on zone 3, and the landing (night arming) is on zone 4.
I haven't room for anything larger where I have the alarm at the moment, though having looked this evening I could move it into the adjacent cupboard as most cables come down the wall past it. If I ever expand too far, the expansion module will have to go in another cabinet in that cupboard, then keybus it to the first cabinet.
Peter