Quite a few but am getting better about it.
At the start I sketch each character on their own canvas according to plan, then scoot it over to the main page where I've scribbled my rough layout and thumbnails. All done with a blue or red pencil tool in Manga Studio. Once all my people and things are the right size and where they should be, I erase most of the crap behind them. Then I make a new folder and add my boxes/panel lines.After that, I kill my first thumbnail layer, double check the position of all my character layers with their boxes, bgs/etc. Once that's all good, I combine them into a new sketch layer.
Inks are all one layer these days. Or I might do a bunch of BG/enviro inks on one and people inks on the other to make last minute fine tuning easier.
All the greys are kinda cell style to fill all those in on 2-4 layers so nothing's touching. (Hair on one layer, skin on another, etc all with reason) Shading is done on multiply.