I mis-typed in my earlier post: it's GroupSummaryQuery that doesn't work for that, not GroupCountQuery. (I've fixed the post)
You actually can get the needed info from GroupCountQuery, but there's a trick:
GroupCountQuery(Album,Disc #) will count the number of [Disc #] values for each album, however, it does NOT count tracks with an empty [Disc #] field. It's not actually totaling up the values of the [Disc #] fields, it's just counting the number of discrete values that exist in the fields, not including null.
So when you filter on the results of GroupCountQuery(Album,Disc #), you'll can get the following results:
0: No [Disc #] is set for any file, so these would be single-disc albums
1: All the tracks have the same disc #, but some tracks might have no disc # at all. These are PROBABLY single-disc albums, you should look at them.
2 or greater: These are multi-disc albums
So imagine the following two-track albums:
Album 1
Track1 [Disc #]=
Track2 [Disc #]=
GroupCountQuery(Album,Disc #)=0
Album 2
Track1 [Disc #]=
Track2 [Disc #]=2
GroupCountQuery(Album,Disc #)=1
Album 3
Track1 [Disc #]=3
Track2 [Disc #]=3
GroupCountQuery(Album,Disc #)=1
Album 4
Track1 [Disc #]=1
Track2 [Disc #]=2
GroupCountQuery(Album,Disc #)=2
You've got enough to do what you need now. Good luck!