Well, maybe not one for 30 dollars.
As for the number of AI racers: 20's would be good, 30's would be awesome. But personally, the complexity of the AI is more important. If there's only 16 cars I'd be disappointed, but if those 15 AI drivers were absolute studs who actually challenge you during a race I'd be willing to forgive a 16 car field.
However, I'm not going to assume that there will only be a 16 car field based only off an E3 demo, I'd just wait for a definitive answer. I think at one point there was a GT6 demo only showing 6 or 8 cars on track and the final game ended up with the usual 16.