Not the OP, but due to neighboring rotor interactions an even number (or a single helicopter like lift rotor and a tail rotor ignoring airfoil lift possibilities) rotor makes design easier and improves performance significantly. Think of the rotors like gears meshed together and realize that an even number spins easily, while an odd number is locked.

Of course there are 6, 8, and larger numbers of rotors used in actual drones. The advantage of more rotors is that redundancy to failure can be built in, and that rotor tip speed for a given lift can be somewhat reduced at the cost of efficiency.