The game doesn't offer any input into how many AIs is appropriate to any given size of map. I've been playing on normal difficulty with sizes like 5 AIs on huge, I also did a game with 8 on insane. Now I keep seeing posts like "oh I'm playing on huge with 27 AIs" and I'm just like...holy crap dude.
I started one game on large with 5 AI and got a total of 3 planets to my name before nothing else was colonizable, AIs didn't seem to fare any better. Colony rush was over before it began, and it felt awful, I like that long period before you meet anyone where you just get to claim a bunch of territory.
Is my problem that I haven't tweaked anything else? I think all the galaxy settings are on default, which is occasional everything. Does everyone bump up habitable planets to common or something? There must be something I'm not getting, because with the settings people are talking about there aren't any planets to colonize.