There is no magic number--it depends on what material, how you make your string, etc. Mine usually run from 11 to 15 (big fingers)depending on the material.
Like Lee stated, counting how many you take out to get the bundles straight after making the first loop will get you very close, if not right on.
Proper backtwist is the "secret" to a nice, round string--especially with a two-bundle string.
Taking all the twists out of the bundles after the first loop helps, but think about it--how do you get those same twists out after you make the second loop? You can't-that's where backtwist comes in.
Is it required? Obviously not--I've seen lots of strings that were pretty rugged looking, and my first attempts were downright awful, but they held together.
Besides looking much better, a nice round string (that looks like one string, not two vines wrapped around each other)seems to wear less, shoot quieter, are easier on the fingers, and may give a tiny bit of increased performance.
It takes a little practice to "master", but only a short time invested per string and can make a big difference.
Chad