Well, I just decided to explore the many fan blade designs out there so you can learn even more about all the choices. The minimum number I could find is three and that is most common on the smaller systems, 1500 CFM to about 3000. You will note that 3 is also the standard for almost all HVAC condenser fans in homes. 3 blades are very efficient and, of course, least expensive compared to more blades.
As I have said elsewhere, Triangle Engineering makes the best hall ceiling mounted whole house fans and they standardized on 4 blades for nearly all their systems from 24 inches up to 48 inches.
There are a few sources for 6 bladed fans. FanMan in Sacramento is one of the first to introduce 6 bladed fans as a unique way to set their products out from competition. They paid Triangle to tool up their special 6 bladed fan so Triangle will not sell it to anyone else in the US. It is interesting to note that Triangle is also the source for 6 bladed fans for Breezepower in Australia.
There are a few other sources for 6 bladed fans. One is http://fresnowholehousefan.com/, another is http://mygreenhomedesign.com/. Both of these companies offer what they call the Zephyr blades. It appears that those companies buy their fans from a 3rd party, thought to be Air
Vent. Note that these models have bolted motors and the belt tension is not held constant by springs like the Triangle and Invisco models.