Hmm... tough question. When I work in PDN I usually merge layers together as I go, so it is neater and easier to work with. if I didn't flatten, I would say probably... 3 or 4 layers for the stars themselves, another 1 or 2 to add variation, another 2 or 3 for color, then another couple for texture.. So for just the stars around 8-11 ish, but that isn't counting planets or the star burst thing. So in short, a lot