stock is 2 front and 1 rear.
it's all about surface area but the real gain is that it's slotted to prevent the brake pads from glazing over. cross drill also helps get the hot gases out of there to prevent overheating and glazing the pads.
I, for one, just want slotted rotors, better heat capacity, and better vein structure for less brake fade.
Installation is too easy to do since it's a direct bolt-on (19/21mm wrench/socket to take off the wheels, jack, 14mm wrench/socket for the brake bracket bolts, 8, 10, and 12mm flare nut wrenches for the brake lines, a vice clamp, and a bottle of ATE super blue fluid and you are done).
25 mins per corner.. 2 hour job for one person & 15 min bleed job with two people working on the car.