If you open a Flac file you can see the songs listed as x/y and the disc as x/y but for Apple m4a the "y" part is lost if added. It would be great to have the "Track x of y" and "Disc x of y" added to the m4a files however Apple saves these non-standard tags. Specifically, I'm talking about the "y" part for both tracks and discs for m4a files.
See here:
Try if you can modify the file list either by adding a column for TOTALTRACKS or set the column for TRACK to use as
Value: %track%[/%TOTALTRACKS%]
If I do a Convert --> Tag to Tag --> Set field to TRACK and value to %track%[/%TOTALTRACKS%] and then import into Apple Music I still do not see the "of" field.
MP3tag can only influence the tag data but not the functions in 3rd party software.
I did not say anything about a converter but suggested to modify the display in the column of the file list.
Please check the extended tags dialogue which fields you get when you save and load an m4a file.
I think that the linked thread describes the problem: TRACK is only numeric for mp4 files and therefore does not allow the / in the field TRACK. The data has to be split into 2 fields: TRACK and TOTALTRACKS.
If the player does not pick that up ...
You could try to tag a file in your player and see what it looks like in MP3tag.
Yes, I've tried that and it's just a single number for track and disc (nothing for the "of" fields. I would donate $100 (seriously) if this feature could be added to update the two numeric fields for the track of X and disc of X for mp4 files. If the track is listed as 1/10 then update the two m4a fields and similarly if the disc field is 1/2 then update both numeric m4a fields. I know that might deviate from the development goals of the app. But I am pretty sure some fixes like this were implemented for the date (actually year) in updating m4a files.
I just checked again with an m4a file: just as predicted - I enter "1/2" in the field for TRACK, save the modification and the TRACK now displays "1". But a check in the extended tags dialogue shows a field TOTALTRACKS that has the contents "2".
So, no data is lost, the field is split so that it matches the mp4 standard. All this from the MP3tag point of view.
If your player does not read or display that field, then there is not a lot that MP3tag can do.
Or try it the other way round: use your player and add the "of y" number in a file and check that file in MP3tag's extended tags dialogue to find out in which field the player expects that data.
Ah okay, so that does work. The TOTALTRACKS and TOTALDISCS do get populated and do show in Apple Music app after editing in mp3tag. My problem is that I take the FLAC file, I make changes in mp3tag, I then drop it into XLD where it creates an Apple lossless version - and from here all changes that I made in mp3tag stick but XLD is not honoring the TOTALTRACKS and TOTALDISCS fields but mp3tag editing the M4A lossless file does. So, I'll have to change the order of my workflow. Thanks ohrenkino.
You can still donate .. 
I just did right now. Please verify. I've wondered for a few years now...are you and Florian really the same person???
No, not at all - I am just a forum user like you and me.
Oh, and a very active one at that. Well, I hope Florian gives you a cut of the donation 
And another "no" right here: Forum participation is completely voluntary.
I was only joking. Thanks for all of your help though through the past few years.