I don't necessarily think there's a specific number. I think that as long as you can provide the necessary needs for EACH dog... including individual training and attention, and can properly feed and exercise them, then it's not a problem.
I'm not in agreement with people owning a bunch of untrained, misbehaving dogs all at once. I've been in a house that had 4 out of control dogs... they all barked constantly, they all had accidents, they would fight, etc. To me, that's unfair to the dog and they're suffering. However, another person could live very comfortably and within reason owning 4 dogs. I really believe the first dog should be trained first, because adding a second when your first already has a lot of issues can lead to huge problems and that's when people get in way over their heads, but it doesn't sound like that's an issue for you! So it's all what you feel comfortable with.
