I'm going to guess there will be a limitation to the numbers for each track in GT5, so ideally the maximum possible that doesn't create too much lag in processing or in internet connections.
F1:CE has 11 player online racing but has horrendous "lag-bubble" collision detection, meaning there is rarely any wheel-to-wheel, bumper-to-bumper racing.
Prologue had it perfect, most of the time there was very little lag, although initially with 16 player grids it was a little choppy. Seems 12 players was more realistic for online.
So I'll say I'd be ok with 12 player grids but I would prefer as many as possible, but not if it hinders the quality of the game. Just as long as its more than 4 or 6, as the longer circuits become rather barren with so few numbers of cars.
I'd also like rolling starts changed to 2-by-2 positioning, because currently the huge single queues of cars are really disadvantaging to the players stuck in 15th or 16th. It would be more bearable with qualifying, which is really a must.