Im an animator, so I can only tell you what
happens in my studio. It is rare to get background
with cel because there is only 1 background per
*scene* of animation. So, there are maybe 12-20
drawings (which made into cel) per second of
animation. If a scene lasts for 6 second, there
can be up to 120 cels but still one background.
Thats why they can have so many cels people can
buy. but no background o_0. As for original
drawing, at work we have in separate box after
show is complete because the cameraman shoots cel
only, not drawing. But now, there is only drawings
usually- everything in my studio is paint not on
cels, but by computer. A dying art.

  Hope this help a little bit ^-^

  ~Koji