3 in my parent's house.
I would really like to have 3 (or even a few more) fosters, eventually, but that won't happen in this apartment. If I move to a bigger place, I'd definitely try 2, see how it goes, and maybe even 3 or 4, but that depends on time and space, and probably won't happen for another few years.
If I eventually have a fenced in yard? I think 5 or 6 would be a good number. I doubt I could handle more than that.