[tirod]

The short answer is four each corner.

The long answer is what is this car doing 85% of the time? If it's a track car, then full on race brake are the issue. If it's a street car with some track time, then donor brakes could be done - PBR fronts all around, with good pads.

As for rotor diameter, it depends. Larger wheels are actually the result of huge rotors, and that implies a lot of high speed braking to nearly a stop. That would be road courses with long straightaways and sharp turns. Tuning the chassis could improve things a lot, tho, step up the G force it can tolerate, you don't have to brake as hard.

There is a lot of interrelationships with the other chassis factors and how it needs to handle.

So, what is this car doing 85% of the time? Get that numerically expressed as accurately as possible, then let that guide how to build a brake system, rotor size, number of pistons, etc. Don't let one feature be the most important priority when in fact it's only one small part of the overall system design.