Ideally, you want a two blade prop - just as a long, thin wing is more efficient, a long propeller blade is more efficient. But at some point practicality comes into play. Long blades require greater ground clearance (the 'gull wing' on the F4U Corsair was to get ground clearance without needing huge landing gear with it's big prop), long props tips go supersonic sooner (noise and loss of efficiency). Adding blades means you can deliver more power to the airstream, and go faster without supersonic tip speeds, but the shorter blades are less aero efficient and you lose efficiency due to the interaction of the additional blades as they pass through the leading blade vortices.
Modern, 'swept' blade props have lessened the penalty of additional blades, but you can't eliminate physics.
I've found it personally interesting that wind turbines have pretty much settled on a three blade configuration. With wind, I figured more blades the better since you're not as concerned with efficiency as much as over all power absorption - but apparently the additional cost and weight of more blades offsets any power increase from adding more prop blades.