4 is the most comfortable, but I play with 8 for a challenge (be it finances, no grim reaper, no social worker, etc). Though I don't do that as often after Sims 1 (before I had cheats or mods).
I think after 4, I start to slack. Someone doesn't get the most optimal skill building or is just a painting slave, etc. And of course babies and toddlers make everyone else take a back burner for a bit. Children can grab leftovers (which are always available), put themselves to bed, shower on their own, etc. But the little people wake up everyone at the drop of a hat haha.
