It depends on the class, the LMP classes don't need any road cars, the GTS class does, you need so many cars made to enter that, they have to be similar to the race versio, same height and I'm not sure on the rules for drive train, the race cars have to be 2wd, but you see cars like the Murcielago which is 4wd being turned into a Rwd GTS car, but there is a Rwd Murcielago on the way, but the race car will race before that arrives. The number of cars to allow you to enter GTS class is 25 of the road version of the race car but if it's close to the 25 mark which usually means the car was built with racing in mind like the Maserati MC12 there are certain restrictions made to level up the field. The old Dome that raced was under different rules to todays LeMans, then you could race a car if he road versions were on the way, but the Dome failed as both a race car and a road car, now it races under the LMP675 class I think.