Well found out this today..

Forza 4: 26 circuits 400 cars
Forza 5: 14 circuits 200 cars.
"Turn 10 places an emphasis on quality over quantity for its next-gen debut.
Casualties include Suzuka, Infineon Raceway and, perhaps most notably, the Nurburgring and its famously torturous 12.93 mile long Nordschleife configuration."