Good thread...
Number of blades is an interesting problem, and i suspect there is much more to it than just aerodynamics...
In theory as long as the rotor has the same solidity ratio, it produces the same lift at the same Nr for the same induced power. Since more thinner blades have approx the same wetted area the total profile drag will be the same for both rotors. Since the thinner rotor has less overall depth there is an apparent weight saving, since each thinner rotors would each carry less load. However, with stress being inverse square of section depth then the need for thicker walls makes weight ends up the same
In practice each blade will have it's own free resonant frequency, affecting the effective hinge offset. There is a need to make sure that the rotating blade frequency is not overly excited by the aerodynamic loads. There is also a need to make sure that the blade modes do not occur in phase, to transmit vibration to the hub. In newer machines this must be the case over a wider range of Nr. Finally the cost of assembling many blades to a rotor dictate that the design team use as few blades as will work.
Mart