2-blade propellers are somewhat more efficient in general. If these variables remain constant, the efficiency of a propeller decreases as the number of blades increases. However, when engine power grows, more blades are usually necessary to properly use the extra power and generate thrust. 4-, 6-, and 8-blade props are common today. The advantage of using a 2-blade prop is that it's cheaper to manufacture and easier to maintain. A 4- or 5-blade prop will work just as well from an engineering point of view, but may be more expensive to produce due to more complex manufacturing processes and materials required. An 8-blade prop will typically have thicker blades which are harder to make and stronger but also cost more to produce. There is no real advantage to having more than eight blades on a prop unless you're making a custom-built prop. Most aircraft engines can handle up to 12-14 blades without any problems - larger props tend to come from turbine engines rather than piston ones because they require more torque at low speeds. However, there are some disadvantages to having more than eight blades on a prop. First of all, it gets harder to make each blade fit accurately on props with more than eight blades. Secondly, more blades means more weight, which reduces efficiency yet again. Finally, making sure all the blades are balanced correctly becomes more difficult as the number of blades increases.