I think the problem laid with Dirt Rally being essentially a better WRC game than the actual WRC games. And instead of trying to make a better game than codemasters they instead threatened them with legal action if they didn't stop using wrc related things like the real stages and the wrc cars.
I cant explain why you cant run 6 but in World Rallycross they don't do races with more than 6. Heats are usually 4/5 drivers. Codemasters are following the same rules which is why you don't see 8 racers at once. Having more races would probably turn into a mess on the tracks.