It depends on what you are making. If you're working from a pattern, it will usually specify one or two strands, or worsted or bulky weight yarn. A single strand is fine for many bags, hats, and mittens; for oven mitts or most slippers, you'll need two. For my felted fedora I used two strands of Peruvian Highland worsted in slightly different colors for a heathered effect. It felted beautifully to a smooth fabric (my hat shrank a bit more than I would have liked. Oops.)