Between our 10 factions (11 with Morgawr(12 with Mezari)) WHo among them should I prioritize expansion with the most? Obviously the cultists cannot expand at all, but among the other factions (specifically Forgotten and Allayi) what is an optimal number of cities? Should I avoid capturing enemy cities if they put me over that number. Beyong disaprooval, what are the disadvantages of expansion?

most of the time I go up to 6 cities for the begining and expand to supportable size on any empire (except for the cultists of course) like KPEMEHb said the only factions I keep small empires with are the argent mages and the forgotten and the allayi, those three are because of different reasons the argent mages because you want to keep your pillars up, the forgotten because you don't want other people to find you, and the allayi because they grow to slowly to make multiple cities worth it.

There is no special benefit for Ardent mages from expansion. You are unlikely to have too many pillars (they become too costly) so as to need more cities to fit them around.

Depending on how you expand the city, I would say it's quite possible to have multiply pillars on one city. And for benefit - more science? Especially if you develop the city? But then again - Ardent benefit from high approval, thanks to bonus for dust and science that both can be used.

That was kinda my point, you can fit 4+ pillars around one city, you don't need too many cities for your pillars.