I have 8 basic layers on every page for the art itself, two for the word balloons, and as many as necessary for the dialogue itself. I typically have 2-4 additional layers for special effects or other details specific to that page. So I suppose, excluding the text layers, I probably have an average of 13 per page.
I cel shade my pages, so it's easy to keep things on the same layer clean and separate.
Basic layers:
Panel borders
Ink
Foreground shading
Foreground flats
Background shading
Background flats
Sketch (hidden after inking)
White canvas fill