I usually try to keep it so I don't have to scroll through them, I used to use a lot more, but these days I try to keep it to a minimum (although that can be difficult!). In my most recent piece I have 6 (although I did recently merge some).
Depends, if we want to create a photo manipulation or the like, sometimes I just use 5 to 10 layers. But if you want to create a digital image or the like, can exceed 10 layers. Essentially, 10 to the upper layer that is enough.
Depends on how many colours and/or elements are in the piece. I usually have one for the base colours and one for shades grouped over top of it at least, so usually about 50, but that number can jump around drastically depending.