there is no simple search string, that's why you can't find it
you'll need to set a variable (i always set mine in a smartlist), then in your view scheme, or in the smartlist view scheme, set an expression that matches the variable
in the smartlist, edit smartlist > import / export, and paste the following: (note, this expression also pulls # of tracks from each disc, i didn't want to separate them and mess the code)
[=save(0,v_tracks-[album artist (auto)]-[album]-[disc #])1]=1 [=save(math(1+load(v_tracks-[album artist (auto)]-[album]-[disc #])),v_tracks-[album artist (auto)]-[album]-[disc #])1]=1 [=save(0,v_discs_[album artist (auto)]_[album])1]=1 [=save(math(max(max(0[disc #],1),load(v_discs_[album artist (auto)]_[album]))), v_discs_[album artist (auto)]_[album])1]=1 [Media Type]=[Audio]
now either in the view you want, or in the smartlist, make a new column with "expression" and paste the following into the expression editor:
firstnotempty([Disc #,0],1) of load(v_discs_[album artist (auto)]_[album])
and if you want the # of tracks paste another column with this
[Track #] of load(v_tracks-[album artist (auto)]-[album]-[disc #])
the results of the above, will be a view (column) that will say 1 of 1, 1 of 2, 3 of 4, etc. so it extracts the # discs and then concatenates "disc #" of "# discs"
if you just paste
load(v_discs_[album artist (auto)]_[album])
it will just show the # of discs in the "set" which is based in this case on album artists (auto) combined with album name. i haven't done this exactly yet, so i'd have to play with it to make the exact thing to show that you want
In fair credit I'm 99% sure I got the original code from mr C.