more cars means more resources, keep that in mind
See my post below.
Based on the computing power leap from the PS3 to the PS4 it is not too much to expect. GT4 had 8. GT5 has 16. GT6 having 32 would probably be a stretch but I think an 8 car increase is not unreasonable. Especially if they can use some good LOD scaling. If you had 24 cars on screen the 20-24th cars would be pretty far away and would not have to be rendered at max detail.
I would like to see a system that allows for a theoretically high number in a race you just cannot see them all at once on screen. Having 100+ Nurb 24hr race would be epic. This would not work for online (100+ connections... PD... lol). But for SP it would still be great. Cars 1-24 are rendered at all times but cars 25-100+ are invisible ghosts during gameplay. As you move within the track, the cars that are ghosts are determined by your relative position to them. So you go from position 25/100 to 24/100, the rendered cars change to the 13 cars positionally in front of you and the 10 cars positionally behind you. The Ai and actual data is still calculated for all the cars though.
When you go to the replay this data can be accessed. So if you want to watch an AI race that was happening 40 positions from you, you can.