On PS3, I started out with 2 sims, but found that I was fast-forwarding a lot rather than actually playing.
When one Sim was at work and the other was reading, writing, painting, etc, all I could do was speed it up until one of them was free.
Now, I play 6 most of the time and rarely get bored or have to fast forward (somebody always needs something, heh).
On the flip-side. My buddy plays 2 (or 3 at most) and loves it. It really all depends on the player. I'm an "achiever" while he is an "explorer". We each get our jollies on our own way.