Ok. For what it’s worth, disc subtitles coming from MusicBrainz never have the number in it (and I personally never tag it in disc subtitle, since it’s a redundant information with the actual disc number).
For albums with many discs, i find it useful to know it, and it’s not like it’s taking much more space, like it has been implemented in Navidrome.
I can’t recall seeing the discnumber in a disc subtitle either and agree that it would be nice to (optionally) display the discnumber beside the disc subtitle for large multi-disc collections.
Considering how niche the usage of disc subtitles is, it shouldn’t be a priority tho.