I use MP3Tag and foobar2000 as a team. I wanted to have foobar2000 show the disc/total discs and track/total tracks. After much Googling and searching for scripts and script components in both the foobar2000 forums and the MP3Tag forums, I realized that I would have to manually set the actual total discs and actual total tracks.
In MP3Tag, in the Tag Panel, I created two new entries: Total Discs %TOTALDISCS%, and Total Tracks %TOTALTRACKS%.
Then I created a new Action called "Fix Total Discs Total Tracks.mta", with the following data:
In MP3Tag, select the files in a single folder/album; first one disc, then any additional discs. In the Tag Panel, based on information either from the discs or from other sources, set the total discs and total tracks for each disc selection. Then select the entire folder/album, and run the Action. Save the changes.
In foobar2000, create two new columns: Total Discs %TOTALDISCS% and Total Tracks %TOTALTRACKS%. Change the Track column to [%tracknumber%/%TOTALTRACKS%], and change the Disc column to [$num(%discnumber%,1)/$num(%TOTALDISCS%,1)]