Hello,
Well, most pictures used for a produce based
on an anime are done with an hanken cel. So,
yes, there can be quite a lot.
Naturaly, other technics may also be used like
water color or CG.

It's very uncommon that a studio will put on
the market an hanken cel as it may need to be
re-used to make a new produce. Also, many hanken
don't belong to the studio but the the company
that made the produce.

Cheers,
Yann Stettler