I left my baking sheets completely unlined. I found that most of the chips came off perfectly, although the ones that were a bit too thin tended to stick. You may want to try with parchment paper, but I can’t guarantee its effects since I haven’t tested it myself.
You can obviously do as many apples as you like, but I found that one apple covered about two baking sheets, and that is as much as I can fit in my oven. The best varieties to use are those with a firm flesh, such as Golden Delicious or Granny Smith.