Movie studios are not required to release the 
cels once the animation is finished. The studio 
can just as well dispose of them in the nearest 
trash receptacle instead of letting the fans have 
a chance at owning a piece of the animation. In 
some cases, a studio cannot release the cels due 
to contractual obligations. Therefore, it is not 
necessarily true that you will find lots of cels 
for a particular show just because lots of cels 
were used to make it.

---Trebuchet