How many playable Sims can you have in a NH?

Though there is no cap really on how many Sims you can place in any neighborhood, you might want to remember (if coming from TS4) TS2 Sims have memories for everyone they met, and everything they buy. This includes all the 400+ townies that the EPs add and the base game.

On lower end pcs you may notice some lag or slow to scroll when your Sims and all the others have visited owned business lots, because even townies retain the stuff they buy in their inventory. Unlike TS4 if you played those these townie Sims you would find the stuff in inventory and all the memories other than first kiss/first woohoo. The game doesn't make them hold that memory, even if they were dating your Sim, when you make them playable later. Point, some people may see some lag or slow to scroll when their Sims they play visit many community lots and meet lots of Sims (they retain those memories) and or run a business and meet just about everyone who comes there, and or buys a lot of stuff at other Sims stores. This goes for townies, too. So, memories, and inventory for everyone in the game may be a good reason to hold down townies population on machines such as a laptop. And maybe not run as many owned businesses because townies keep those memories, too, and all the stuff they buy.

Legacy houses/lots hold all the data of every Sim that lived there, and though you can play a legacy generation after generation, just remember that lot has all the left over data of all the Sims there, and maybe one day you should move a family to a new, unplayed lot.