Arguments for the choice of the number of blades B are given in chapter 4.3.2 and chapter 5.2 of my report KD 35. The higher the design tip speed ratio, the lower the number of blades has to be chosen. To my opinion, for a fast running rotor and a direct drive generator one has to make a choice in between two or three blades. Both options have certain advantages and disadvantages. I have chosen for two massive wooden blades for the VIRYA-5 rotor connected to each other by a flexible steel twisted strip. No welding is required as it would be the case for the spoke assembly of a 3-bladed rotor. The rotor has a constant chord of 240 mm and a Go 711 airfoil with a maximum thickness of about 34 mm. It has a constant blade angle of 6 degrees and so the blade isn't twisted. Manufacture of a blade is therefore rather easy.
If I would have chosen for three blades, the chord and thickness would be only 2/3 of the chord and thickness for a 2-bladed rotor of the same diameter and design tip speed ratio. So the chord would become 160 mm and the thickness would become 22.7 mm. The bending and torsion stiffness goes down with the power of four of the scale factor so it goes down with a factor 0.198. These blades will be much too weak and too flexible and will be very sensitive to flutter. Another disdadvantage of the smaller chord is that the Reynolds value for a certain wind speed goes down with a factor 2/3 and this results in increase of the airfoil drag coefficient. The slightly higher maximum theoretical Cp of a 3-bladed rotor above a 2-bladed rotor (see KD 35 figure 4.3) will be neutralised by the higher Cd/Cl value (see KD 35 figure 4.6 and 4.7). For a 3-bladed rotor with constant chord blades, one has to chose a lower design tip speed ratio in between 5 and 6. However, this would result in a too high design wind speed in combination with a 34-pole generator running at 50 Hz (see KD 614 figure 5).
The main disadvantage of a 2-bladed rotor is that the gyroscopic moment in the rotor shaft is fluctuating; it is maximal for the blades vertical and zero for the blades horizontal. The gyroscopic moment is proportional by the product of the angular velocity of the rotor, the angular velocity of the yaw movement and the moment of inertia of the rotor. The hinged side vane safety system has a large moment of inertia of the head and this limits the maximum yawing speed and so it limits the gyroscopic moment. The fluctuation of the gyroscopic moment is decreased if the blade connection of the blades to the hub is made flexible. This is the reason why the connecting strip is rather thin (10 mm).