Use minimum 4X the draw weight of the bow. What is the tensile strength of FF? If it is 50 lbs, then a 50 lb bow uses 4 strands minimum.
With Dacron B50, I am using 8 strands for a 50 lb bow and having to double serve the arrow nock area so that it doesn't slice thru my fingers! I've never had a single strand of it fail in a bowstring. Remember, when you start adding weight to the bowstring, performance suffers. FF is supposed to be stronger than Dacron, 12 strands would be overkill in my opinion.
But then, I think FF on wood bows is overkill period. I'm too cheap to gain 4-5 fps for four times the price of Dacron.