Racing motorcycles would be unfair mixed with cars, because acceleration on a cycle is faster than a car. I believe on all the screens of the real GT HD we've seen so far only have cars on them, but Kaz and the lads could surprise us. Still, bikes would only work in bike-only races.
As for cars in a race, they are intending to have up to 20 cars in online road races, 8 in rally, and the same offline. If they had 12 road and 6 rally, that would be good enough for me. That's what they had running in the E3 demo.