REM, which tire is best for a chevy? haha.
It is really whatever you want. How many would be strong enough for #50? I'd bet 6 would hold it as good as 20, it would just stretch more, etc.
There are 2 very important components of a bowstring if you ask me, ample thickness in the loops and the proper serving fit.
Other than that, it is personal preference.
If I were you, I'd go with whatever number would make your serving work. I'd bet around 18 would be good, but don't know what serving you have.