My biggest cel is 7 layers thick!  Its really 
amazing!  I dont have an image readily available 
to throw up here, so here is the link to the 
cel :)

http://www.geocities.com/bassanimation/pages/FYSho
kaMiaka.html

It's an FY cel, and it's just gignatic!  There 
are all kinds of layers for the airbrushing, 
mouths, and other various things :)  Truly 
astounding!  Cant wait to get it framed so it can 
have the display it deserves, enjoy!  

Woops, here is an image I found!