Is it more efficient to have many or a few rotors?
Rotors need something to power them and the current generation of aircraft (once you get bigger than model size) burn hydrocarbons to get that power. If you want twelve rotors, then you either need twelve engines or you need a smaller number of engines plus a bunch of heavy driveshafts and gearboxes.

I assume from the fact that most contemporary aircraft designs use a small number of engines (usually one or two; sometimes three or four) that having a few larger engines is generally more efficient than having more smaller engines. This seems to apply equally to jet engines, turboprops and piston engines.

It's possible that the tradeoffs will change if you move to electric motors. However, at the moment, batteries are so heavy that electric motors don't seem to be a practical option.