It depends on how complex the page is, and how many sketch-layers I end up needing.
The average breakdown is thus: 1 layer for panel borders, 1 layer for rough stick-figure sketches, 2 layers for text and word-balloons, 1-2 layers for clean sketches, 2 layers for inking (one for figures, one for environments), 2 layers for flat colours (figures/environments), 1 layer of atmospheric lighting adjustments, 1 layer for shadows. If there are extra lighting elements, an extra layer for those.
So, 12, if it all works out okay. It can go all the way up to 20, if I'm doing crowd-scenes, complex lighting situations or sound-effects. All the adjustment layers - lighting and shadows - go on top of the inking layers, since I colour my lines to make them blend better with the flats, and putting them underneath the adjustment layers helps them blend better.