I have yet to understand this one completely.
Somewhat guessing here but I assume it's most likely due to the fact that on PC, you can turn your graphics settings down to a level that can handle many more cars on track at once.
Edited to add: Forgot to mention that on most PC games I've played, you can set the amount of cars you can "see". Meaning that even if there are 64 cars on track, you can set it so that you'll only see, for example, 24 of them at once. This works great because it'll save on system resources, and really how many cars to you need to see at once anyway?
Plus you'd only ever see that many at the start of the race. After a lap or so, you'd most likely never be around more than 20 cars or so anyway.If they lowered graphics on consoles to that level needed, everyone would scream how shitty the game is.
For online, it's most likely the same deal. Plus dedicated servers. Lots of leagues rent 100mbps servers from sites like Gameservers.com, which allow for a significant amount of players. If everyone in the room lowers their game settings a bit, and they have that amount of bandwidth on the server, I can see how that could work.
Is there an on-line number yet?
I think they're shooting for 64 on PC. That's for both offline & online.
Nothing official that I've seen for consoles yet.
This is another reason I'll likely get both PC and PS4 versions. Mixed class racing at Green Hell with 64 players is something I've been waiting for for a long, long time.