Mazda had 3 basic rotor designs, The 10A, 12A and 13B. They are all 2 rotor designs and the main difference is the width of the rotor housings. All the early motors used side intake ports but the race rotors used peripheral ports where the intake is straight thru the side of the rotor housing.
The 20B 3xRotor is just 3 of the 13B rotor housings stacked together. In theory you could make one 4, 5 or 6 rotors long if you could machine up an excentric shaft long enough (What a rotory has for a crank shaft). The problem with the longer rotor combinations is getting the heat out of the center of the motor. They do this by pumping large ammounts of
oil throught the excentric and using big
oil coolers.
Mazda won lemans a few times with a 4 rotor motor but I don't think it ever went into a production car. It was an awesome machine and pioneered variable length intake runners for max torque across the rev range.
They've got a 4 rotor supercar concept out at the moment. It'll be interesting to see if it hits the show rooms.
Cheers