There is a reason for that.  Typically fewer blades run faster and more efficiently.  Two blades runs (as a general rule) a bit faster - and even more efficiently in many cases than three but the drawback is vibration.  (they shake when they yaw).  Sometimes the shaking can be overcome with weights - or sometimes it may seem un noticable, but any time you have two heavy spots a wheel and turn it (like a windmill yaws) its going to shake.

More blades generally have a bit mroe surface area and start more easily. 3 blades is a good compromise, probably the best compromise.

"I will be building an alternator that will need a twelve foot three bladed prop. Can I use, say (just for discussion) a six bladed 6 foot prop? "

No.  Your 3 blade 12 footer would run at a reasonable speed and probably have lots of power.  Your 6 blade 6' would probably have too many blades -  a bit mroe torque - but it would be inefficient - even for a 6' prop.  Power is related to the square of the diameter (the area).  Your 12' blade covers a bit over 100 square feet.  Your 6' prop covers about 30.  The 12' prop (done well) should make 3 X the power of a good 6' prop.  A 6 bladed 6' prop will have lots of xtra drag, so itd be even less powerful I suspect.

the 6 bladed 6' prop might have a bit of xtra torque to get your big alternator turning, but it will not have the  power to spin it up to the sorts of speeds itll need to go to make power as it would if it had a 6' blade.