At the moment I have a family of 6 (parents and 3 kids and 1 IF) and each of the kids has their own room (except 1 shares with her IF) but this is the same house that a generation earlier I had 8 sims (granma had own room, parents had own, eldest and youngest shared a room and triplets shared a room)... Its all about the size of the room and how many single beds or bunk beds you can fit in it!