Dredging my brain for what I learned all those decades ago in aerodynamcs lessons, I recall that when increasing the number of blades on a prop hub i.e. increasing 'propellor solidity', there quickly comes a time when the turbulence off of one blade impinges closely on the following blade etc, and aerodynamic efficiency starts to reduce.
Modern fluid dynamics computation, and simulation, has very likely allowed designers to very slowly increase the number of blades in a prop design, but there must surely be a point where, even with incredibly fine tolerances in manufacture, where it really is just impossible to (economically) get one over on mother nature!