Most area would be okay with 2 cats. Here in NY for instance(exc. NYC), i can get my car pass witout cats at all. some requires some cats. Some area, like where I'm moving to in ohio, if car came with 4 cats, then must have 4 cats. and it have to be same distancve from engine, otherwise will fail the sniffer test, because farther away fron engine, slower cat lightoff....