Personally never count my workers, generally just pump until I'm rich enough to sustain a whole base, a lot of times its too much and need to get rid of a stack or 2 mid-late. It's been said 50 is the max amount of workers the patch can be mined at a time since I can remember but Ive never tested it, wonder how true that is.
Also aren't there 2-3 patches(stacked) on some maps, which I guess would mean u can do 50xwhatever the amount of patches? That would make sense as to why sometimes minerals dont run as smoothly as they should. I really don't know but perhaps someone can clarify.