Guys, it's worse than you think!
Problem #1
In a Darrieus turbine, at no time are the blade forces in balance. The advancing blade is always seeing more relative wind velocity than the other, and the lift forces are therefore higher.
Problem #2
Due to different relative wind velocities on the blades, they are also never at the same angle of attack.
Problem #3
Due to #1 and #2 the direction at which the lift forces are applied there is always one blade with more mechanical advantage than the other.
Problem #4
As was pointed out above, it's more difficult to balance with all the weight of the blade concentrated at the outside radius.
Problem #5
Moment of Inertia is a property of rotating objects. Apart from its effect on speeding up or slowing things down (which isn't really a safety issue), it has a strong effect on the energy stored in the rotor at a given RPM. A typical HAWT with 2 blades has a lower moment of inertia than a VAWT, because the mass of the blades in the VAWT is concentrated at the outside of the radius (whereas most of a HAWT's mass is near the hub). When (not if) something fails, there is more energy unleashed on the innocent bystander.
Problem #6
Similar to #5, the structure of a HAWT (especially a H-bar Darrieus) wants a slender arm. These spindly things must absorb the energy of all the things listed above. On the other hand, the HAWT's hub is the beefiest part. Tapering the tips is both aerodynamically and structurally efficient.
None of these problems are resolved by changing the number of blades.
Unless you are sure that you are competent to safely and reliably overcome these limitations, for whatever gain in captured energy, then step back and find someone who can evaluate the loads and the safety of the machine.
I'm an engineer myself, and when I saw how my 1.2 meter machine behaved, I went back to the math I did before building it. Seeing is believing, and it was time to take it down. Particularly when I think of how my 5-year-old likes to watch it run.
Think about it...