Unofficially, there are 2 bathrooms for 4 rooms (with 2 people per room, so 2 bathrooms for 8 residents). I say "unofficially" because you don't necessarily have to use the bathroom closest to your room. The two girls across the hall when I lived in McKay tried to usurp a single bathroom for themselves, but thoroughly failed
Look at
this image of the third floor layout for instance. The 4 rooms in the red box (306, 307, 322, 323) share two bathrooms amongst them (321, 324). And no, you do not have to clean them. You should try to keep in clean, but the cleaning staff do the actual cleaning.