There're a good amount of hanken cels make for 
each show, and for each of its merchandises.  Such
as DVD, CD booklet, pencil board, poster,
and magazine covers...etc.  The reason that you
rarely see hanken cels because most of them are
kept by the staffs who work on the shows.  Some of
them are kept by the studios, they don't want to
sell them like the production cels because of 
their quality.  Some studios like to display them
in some conventions or events, that's why they're
keeping them.  And some of them are licensed by
the manufacturers, like the pencil board 
manufacturers hold the rights to the hanken cels
that are made for the pencil boards.