It depends on how well your computer runs. More sims will be harder on your computer.
I don't know the definitive number off the top of my head.
Yeah, I was wondering what it was a function of. Available RAM isn't what I would have guessed, though. I got error code 12 a couple of times way back with an inferior computer and again with a virus that sucked up RAM, and there doesn't seem to be a correlation between processing speed and move-outs.
I've heard many implications regarding what this is a function of, including number of homes available, balanced demographics, and straight up hard numbers. I'm also not assuming that there's only one variable here. Not including NPCs and townies, I have 116 people in this very big town. If I plug it in for the first time and thirty people move out, I'd be disappointed.