The big snag with 2 blades is the vibration during yaw. Any scheme with a tail suffers from this. With pitch control and servo yaw the problems of 2 blades are much less and for small machines it could be acceptable. It should be easier to hold 2 blades in step rather than 3 for a pitch control mechanism.

If you need to go to a smaller diameter for 3 blades then you need to do the same for 2 blades.

What matters in the end is tip speed ratio. If you were desperate for speed you could push 2 blades to slightly higher tsr, but I would not try for tsr higher than 7 for home made blades and you can manage that with 3 blades.

I am not really sure what performance you can get from planks, the ceiling fans seem pretty useless and no commercial machine has ever adopted such things. I would imagine that if you use a slower and more costly alternator and worked at tsr4 or below you might get some results, but even then they are less strong for a given weight than sensible blades. I don't think they will be comparable to pretty poor constant pitch constant width blades made from pipe, the curved surface of these does seem to work better in lower winds.

You can pull all the power pullable out of the wind with ANY number of blades - even one (with a counterweight for balance, like a pinwheel made with a maple leaf with a pin through its center of rotation on a stick).  This is because blades affect wind both upstream and downstream, for a considerable distance.  If the next blade-pass occurs about the time the slug of slowed air has blown downstream by its own effective thickness, you get it all.

Now this means that, for a given TSR, the fewer the blades the wider they must be to affect a deep enough slug of air.  But at a TSR of 6 or so even a two- or three-blade mill has very narrow blades when it's sized to get all that's gettable out of the wind.

One-blade has lots of problems.  (A big one is that it's hare to get the center of drag to match the center of rotation.  So it really shakes the mast.  Another is the yaw vibration - see two-blade.)

Two-blade is pretty good except for the yaw vibration problem:  A mill is a gyroscope.  Turn it one way and it tries to tip back, the other way and it tries to tip forward.  With a two-blade the angular inertia varies sinusoidially with blade angle.  Yawing it while the blades are horizontal makes it try to tilt HARD, while they're vertical it doesn't make them try to tilt at all.  A yaw takes many rotations of the blade to occur, so it vibrates hard while yawing - and windmills spend most of their time yawing to one degree or another as the wind constantly shifts.  This effect goes away when you have three or more blades.

So three or more blades are pretty much fine in all respects.  More than three blades means more blades to construct, shape, mount, paint, balance, etc.