The traditional bow hunters hand book by TJ Conrads says up to 18 strands for a flemish twist. B-50 recomended strands are 16. but I think he needs to up date his book too.
almost all the jigs I have seen have the capibility to wrap 9 strands. useing two 9 strand bundles to make an 18 strand string.
I know that a lot of FF string makers are useing 12 strand strings for the more high performance bows. 6 strands for a 2 bundles twist. I use three 5 strand bundles to make a 15 strand string doing a three bundle twist.
hope that helps.