B50, Brownell brand, was said years ago to have a breaking, tensile strength of around 45lbs.
By this, 8 strand would suffice, but the issue than becomes the "shock" strength, and stetch!
I would suggest 12 strands, but if your happy 8, that works! Being that your making your own, you can keep an eye on it, and fix any issues, as needed.
Dick is right about the adjustment for different diameter strings. I have said this before to folks who want to try making "skinny" ones. It's like the ratios of gears. Your measurents will change.