uuuhhh... its 12-14 strands for #50. x3= 36-42 strands... i would go 45 to be safe. wax it really, really, really well.
go for about 1.5" shorter than the prod cause it stretches.. you could do a continous loop(take a piece of wood, stick nails about 1/2" more apart from the length of the finished string, wrap it 22 times between the nails(44 strands in the finished string, serve the ends with cotton or B50 or linen or something, then twist it up) i have done that for a pistol xbow...
or just go for a super short flemish twist, and splice both ends instead of just one.
Whatever you do... wax it insanely well.