It depends on the page - pages with more complex lighting arrangements obviously require more layers - but I average somewhere between 10-20. Another thing that can drive up the number of layers is how many sketch-layers I need; some things require a lot of sketching. In this particular example - a page from chapter 3 of Grassblades, with a bunch of rimlighting and extra effects between panels - I've got 15, and that's because I've kept my sketch-layers in the file, although invisible.
I do my inking for the backgrounds on one layer, and the inking of the characters on a separate layer - this is because I tend to colour the lines, and like to keep the outlines of the characters darker, and that's easier to do when they're on separate layers. Also, it's easier to ink them separately so that I don't accidentally erase something when I have to fix a line in the background or something.
I also colour the backgrounds on one layer, and keep the colours for the characters on a separate layer - again for the same reason.
I do shadows on a separate layer set to multiply, and adjust lighting with an overlay-layer, and do rim-lighting and extra lighting effects on a glow-dodge layer. I could simply paint all of those on the same colour layer, but that would take me infinitely longer to do.