When I first read this I thought 'surely that's straightforward'.................and then I tried to do it....................not so straightforward.
However I have come up with this (feels like rather clumsy) method using three virtual tags:
1. Create a unique identifier for each disc:
AlbDiscID<Disc#><Album><Album Artist>
2. Assign a number to each track which represents an average value for the expected disc track count (i.e. expected disc track count divided by actual disc track count):
ExpDiscTkCtAv$Div(<Track Count>,$Count(<AlbDiscID>))
3. Sum this average value over the whole album to give the expected album track count:
ExpAlbTkCount$Sum(<ExpDiscTkCtAv>,<Album>)
This definitely needs some more testing as I have only tried it on a system with a very small and simple library, but it appears to work.
I also feel that it might need some handling in the case of any rounding errors - not sure, and I haven't looked at this.
Edit 15/01/24
$Count(<AlbDiscID>) was $Count(<Title>,<AlbDiscID>) - removed unnecessary code producing the same output