Strand counts are often ludicrously high especially for a
longbow.
I use a 'harder' (stronger and low stretch) string, AstroFlite but I've just done a 100# warbow and used just 12 strands.
I typically use 8 for bows upto about 50# then 10 up to 90#
For B50 I'd suggest 10 was plenty for a 40# bow.
The advantage of a low stretch string are a lighter string adds speed, but you need to lay in extra strands or serving for comfort/nock fit.
Note, I do continuous loop so, my 8 strand string only has 4 strands round the loops and thus I lay in a couple extra (but the loops are served). With a Flemish twist you get full strand count at the loops.
Have a play with the 3 strand Flemish twist which gives a more compact and stronger string per strand count (according to what I've read), but of course you need multiples of 3.
So a 3 plies each of 3 strands would give a nice 9 strand string.
I've only done Flemish Twist occasionally, with linen for a nice authentic medieval string. Linen has a lovely feel when made up as a string.
I followed the info in the Traditional Bowyers Bible vol 4 which has a great section on string making, natural fibres etc.
I've recently made nettle cordage and some from rose shoots too, quite fun and surprisingly strong. Hard to get it even and consistent tho'
Del