I dunno about in sims 3, because i haven't heard much about it but if you use cheats in sims 2 you can have as many people in the household as you want. for example one of my family's has around 15 sims in it i think. So im guessing sims 3 won't be any different.
the cheat is (boolprop testingcheatsenabled true) then you hold shift on your keyboard and click on your sim then go to spawn then tombstone L and D then click on the tombstone on the ground and add your sims!
i think you can only have seven, but you can let friends stay over and spend the night if you want more people in a household, but the more sims you have in a household the more chaos so you are probably better off sticking with less people.