A lot of the string noise on endless loop strings on recurves is created by the string slapping against the recurved tips as the string comes back to brace height at the shot. So, it's the material that is wrapped around the fast flight at the nock ends that will determine how noisy they will be on recurves. Nylon would be much quieter than monofilament, for example, and probably quieter than a fast flite material like Halo. On longbows, this isn't an issue.
Don't know how many strands of fast flite you would need to create a string of the same thickness. That would depend on the fast flite material used. But why would you want to do that? The advantage of using fast flight is it's thinner, lighter and stronger so its possible to build a thinner, stronger more efficient string than B-50. Just use a heavier/larger diameter serving material to get the same string diameter at the serving/nock point.