In general, the higher the number of blades the less vibration will be transmitted through the rotor hub. This is judged against minimising the number for cost. You can't stop each blade from flapping & twisting at specific frequencies, but more blades means they all vibrate in phase at a higher frequency.
There is a lot more to this, but for a full understanding you need to look up Campbell plots and symmetric and antisymmetric resonance modes. Just accept that designers try to fit as few blades as the dynamics will allow...