Bonus credits are awarded based on the number of cars in your garage in Forza Motorsport 3, Forza Motorsport 4, and Forza Horizon on the Xbox 360, and Forza Motorsport 5 and Forza Horizon 2 on the Xbox One.

Any game with 50 to 199 cars in your garage
Will award 100,000 credits.

Any game with 200 to 299 cars in your garage
Will award 1,000,000 credits.

Any game with 300 or more cars in your garage
Will award 5,000,000 credits.