Multibladed have a smaller diameter disk so may have lower tip speed hence less issues with retreating blade stall, disymmetry of lift and supersonic tip speeds and hence allow faster flight speed.
...until they start getting big (and heavy). Then you have to reduce RRPM to keep tip speed under control... and once you do that, you start running into disk loading issues and total blade surface area which increases chord width (for a given diameter) and so on. Imagine a heavy helicopter with two blades and you can see where da Vinci got his ideas...
