I hope I'm wrong, but I read something like 2 bikes on the scan (not the picture, but the text)
Don't know about the rest.
Maybe in some race or something will you reace head to head......
Watch the TGS trailer. It has some of the most advanced physics in a motorcycle game. From that video (no ingame, only a post-AAed replay, just like in the major part of ps2 GT intros) it's better than Namco's MotoGP4, THQ Motogp 3, GP500 (extremely hard & fun simulation, but the physics were not that good), Superbikes, and anything I can remember now. Just look at the motorbike reaction and the pilot animation, it's wonderful.
And, do you remember when in the making of GT4 we saw motorbikes? Well, here is the answer That's why it can be released so soon IMO. A lot of work is already done. Great planning, polyphony.