The number of blades determine the solidity ratio of a propeller. If you need to limit tip speed and propeller diameter, the propeller will have a high solidity ratio by using many, short blades. Take a ship's propeller: It needs to work in the space between the draft of the ship and the waterline, so it will have many stubby blades.
If space allows, a bigger propeller has a better efficiency because it can use a larger mass of air for lift creation. If the propeller has a low advance ratio, the wake of one blade might interfere with the next, so packing more blades will increase the risk of interference. The fewer blades you need for lift (or thrust) creation, the better. Efficiency generally runs inverse to the number of blades.
A single bladed propeller or rotor needs a counterbalance, which in effect is a dead weight. When the interference between two blades is low enough and the disc loading of the rotor is not too low, a two-bladed rotor will be the better choice. Disc loading is the lift relative to the area of the rotor disc and dictates the chord of the rotor blade(s) once their number is defined. Only when structural limits will make the chord of an ideal two-bladed rotor too short will a single-bladed rotor be attractive.