It's not strictly defined. As it stands, it's a "mega house" if it has more than the typical number of scenes AND Universal decides it's a mega house. Previous mega-houses had about 16-18 scenes. A traditional house has 8-11.
Stranger things is clocking in at 14 scenes from the moment you enter (no indoor facade), so it's in an interesting middle-ground. SS25 is packed though. They used every bit of space they could.