At first, I was rather ambitious with these numbers, dreaming about 64 plus grids in an offline career mode VLN race, with maybe as much as 5 classes. 12 cars in each class with an extra 2 cars in the class you compete in and 2 more in randomly selected class. Now that may sound way too much, but whether this will be possible on PS5 or in the future I reckon that's the closest we may ever get to an actual 160+ VLN grid on a GT game, and have the ability to experience a good portion of the unique multiclass element the Nürburgring employs IRL which makes it so special.
For the sake of balance though I've entertained the idea that the VLN races would be the only races where a grid of 64 cars and 5 classes would be possible. The VLN races and N24H race would be the halo of the career mode.
GT7's career mode could then feature unique races where special versions of the tracks are specifically coded to hold a large grid of cars and support 5 classes at once. Such as VLN with 64+ cars 4-5 classes, Le man's races and championship series calendar venues sub 50 cars and 3-4 classes etc. This would be largely dependent on track length. It would be nice to have consistency in grid numbers no matter the mode but I believe it would be better to experience this calibre of races offline.
A 30 car grid count would be great as a maximum for the majority of the game, but of course it's still a large number and should not be really seen until the mid-tier events in career mode. Arcade mode and online lobbies would allow you to set the maximum grid of 30 at will, with the possibility to set a division of 3 classes racing at once. This should be dependent on if the track meets the requirements though, so no 30+ cars on Tsukuba or Willow springs layouts.
The minimum number of cars I would want to see in career mode races or any other event in GT7 is 12. I just shudder to think how I used to do those hours long races in GT4 with 6 cars.
Great game but the life-like timed endurance races with those 6 car grids were pretty laughable and lonely. GT5 definitely improved on it but not greatly.
Hopefully we can experience some semblance of packed grids on GT7, particularly in the most iconic races/tracks where in real life a large entry list is typical