If I recall correctly, [Total Discs] was added to Media Center 1-2 years ago, so anything imported prior to that will need updating.
I'm not sure whether Media Center's "update library from tags" function will pull that in for existing files - some tag changes require that you remove and re-import an album.
Unfortunately I ran some tests and it only seems to fill this information out if your [Disc #] tag is in the format "2/4" for disc 2 of a 4-disc album for example.
If it's just "2" in a directory with three other discs, Media Center doesn't give you a total.
And if you fill out the [Total Discs] field manually, it does not write any tags to the file at all - even if you use the "write tags to files" feature. It does not use the "2/4" format or store it in a unique tag.
So if you convert the tracks externally from Media Center - or perhaps even using Media Center - that information will be lost.
That said - and it's partially due to Media Center's prior lack of [Total Discs] support whatsoever until recently - I have largely switched to using Media Center's "fill track numbers from list order" tool on multi-disc albums.
In my library at least, I see very little reason for the concept of "discs" to actually exist any more. I don't clear out the [Disc #] tag, but rather than naming files: "1-14", "1-15", "2-01", "2-02" I have tracks: "14", "15", "16", "17" etc.
What I do rather than clearing out the original information is have the "fill track number from list order" use the [Episode] tag. I then use expressions which rename files or sort by [Episode] rather than [Track #] if it is not empty.
I also do this in reverse for TV shows. Rather than "[Season]-[Episode] - [Name]" I use [Track #] as a prefix which is a continually incrementing number rather than starting over again for each season.
As a bonus, this works better for shows which have a movie or two in-between seasons.
For example: Battlestar Galactica (2003) essentially has:
- Miniseries - 2 episodes
- Season 1 - 13 episodes
- Season 2 - 20 episodes
- Season 3 - 20 episodes
- Movie / Special 1
- Season 4 - 20 episodes
- Movie / Special 2
That's a nightmare to try and tag and sort correctly in a single list view. You have to trick it by using intentionally wrong [Season] and [Episode] tags, which I don't like.
What I do is keep the correct information in the [Season] and [Episode] tags for file naming, but use [Track #] as a prefix, which makes managing file sorting or playback order
far easier.
That way I don't have to call the Miniseries "Season 0" or the specials "Episode 21".