One blade would be the theoretically most efficient if not the most practical.
More blades are used when the load on the rotor disk is too great for fewer blades. To generate a relatively greater lift the speed of the advancing tip would get very high compared to the speed of the retreating tip - the 'Advance Ratio'. If this ratio gets out of limits then the retreating blade will stall. Even if the retreating blade doesn't stall then the advancing blade will start to hit Mach Buffet in the transonic regime and then that blade would stall.
To get around this and reduce tip speed, more and smaller blades are used. Their are also logistical and size constraints where more, smaller blades would be preferred over fewer, larger.
