I generally use 2 plies but I have done 3. More than 3 seems a waste of time. (Now that I wrote that... someone will write and say that they LOVE doing 4 plies... I just know it!) I rarely, almost never, work with singles because I like the balanced feel of a plied yarn and I prefer the look of it in the finished garment. That said, maybe I just spin [censored] singles that need plying <img src="/images/graemlins/smile.gif" alt="" /> I don't see any advantage to more then 2 plies though, for what I like.