Unlike cel size, I know layer number has an 
absolute upper limit because of the darkening 
effect of the acetate thickness. My thickest setup 
is also 5-layers (no corrections, the El Hazard 
cel of the scene after Makoto bumped into the 
statue... I'd like to see if anyone has a setup of 
6-layers (and no, I don't mean a bunch of 
corrections ^_^).