Update: All cars in GT6 that made it to GT5, that's 1085 cars (all including the six Chrome Line cars and the six Stealth Model cars)
A complete GT5 had like 1106 cars in total (including the six Chrome line cars, the six Stealth Model cars, the fictional GameStop NASCAR Camry, the Ginetta G4, the two Ferrari F1 cars, the Lotus Europa Special '71, the Nissan 350Z GT4 Edition and the Jensen-Healy Interceptor.)
This could mean we would start Arcade Mode with 1089 cars (adding the Hyundai Click, Hyundai Accent Rally Car and the Audi Auto Union Type C Streamline). As the Honda Fit RS '10 is your "mandatory" first car, it will automatically be added. We'd have to unlock 135 cars to have all the cars in the game in arcade mode.
GT6 now has 1225 cars, but the Lunar Roving Vehicle is not playable outside of Lunar Exploration mode and can't be obtained. On the car list, it says that there's a BMW M4 "Frozen Black Metallic" variant, although it's not in the game as well.
For those with 15th Anniversary content, they will start with 1114 cars in arcade mode.
(C'mon PD and Kaz, you better see this!!!)