It was always my understanding that the ballistic tolerance of a rotor blade was a function of the robustness of the structure. A wide cord blade would be more balistically tolerant than a narrow cord blade. The trade off becomes one of ballistic tolerance versus ballistic susceptibility. i.e. What are the odds of hitting a narrow cord blade versus accepting a hit on a wide cord blade designed to survive the hit. This is where number of blades comes into the equation. The Blackhawk, Apache and Boeing YUH-61 UTTAS all used four bladed rotor systems with relatively wide cord blades in an attempt to achieve a balance between maximum ballistic tolerance and performance. I am sure that John Dixon could add to this discussion.