You can take it a step further and have a counterweighted single blade propeller, which is the most efficient. That being said, this is only aero efficiency. O

You can take it a step further and have a counterweighted single blade propeller, which is the most efficient. That being said, this is only aero efficiency. Once you start to add electric motor efficiency, things get interesting. To generate thrust with lower number of blades you need to have large torque torque to put into the prop , which means the motor has to be a low Kv (Kv = rpm/volt, and a low Kv means a high Kt where Kt = torque/amp), to keep the current lower to lessen electrical power losses. Or use a reduction gearbox. Both of the above necessitate a bigger package and more weight, which requires more thrust, and so on. There is a place in this design space where having multiple rotors is actually more efficient for the overall flight envelope.