Propellers come with all different numbers of blades. In general, the more blades you have, the more thrust you will be able to generate. For a fuel burning engine, this is the only concern. For an electric motor on a sailboat, there are other factors to consider.
The ideal number of blades is an odd number. On a smaller boat, 3 blades; if the boat can fit it, 5 blades.
The reason an odd number is ideal is because this keeps the same number of blades exposed to the water around the keel the same which makes regeneration of power from the motor a lot more uniform.
When a propeller spins behind the keel, the blades exposed to the water will oscillate as such:
2 blades: 2,0,2,0,…
3 blades: 2,2,2,2,…
4 blades: 4,2,4,2,…
5 blades: 4,4,4,4,…
When an even number of blades spin, the propeller will oscillate between all the blades showing and then two of the blades hiding behind the keel.
When an odd number of blades spin, there will always be just one blade hiding behind the keel. This will oscillate between being up or down blade behind the keel, but the number of blades exposed to the water will always be one less, not two, than the number on blades on the propeller.
This means that an even number of blades will oscillate in power while an odd number of blades will remain steady and constant while you sail.
