Tooltech,
it all depends on what you're wanting. If were just concerned with how many strands will hold that amount of weight, I make some 6 strands for people when they order them and stretch all the strings out to over #200. Although that is just a static stretch and different than actually shooting the bow, i think 6 strand bcy x would hold a #55, but i wouldn't really want to do that. It really depends on exactly what you want.
As far as serving goes, there are quite a few good ones out there. One important thing to know about serving is they have different laying qualities. For instance, power grip has a VERY GOOD finish on it and is also round, even after you serve it on the string, it is still round. Halo tends to lay flat.
What really determines the durability of the serving is the material and the finish. If you go to bcyfibers.com you can look up what the make up of each type of serving is and then you'll see what makes them quality.
Power grip is a blend of spectra and nylon yarn and halo is just spectra. Power grip is the proclaimed "latest and greatest". I have been using it for several months and I really do like it. But i like halo as well.
sorry for the long answer