As others have said, it stands for the seating. 2 in the front + 2 in the back. That equals 2+2. Not really for people, unless they are small but it made the insurance companies happy and they reduced the insurance rates because it was not a 2 seat sports car.
it's really for insurance purpose during that time. Sports cars were considered 2 seaters like your vettes among others. 2+2 meant 2 seats in the front and 2 in the rear. Strong car but not classified as a sports.