Thank you for your reply.
I have weird behavior in MP3Tag.
If Picard has Total Discs tag written, MP3Tag will not display the track number in the format disc_number/total_discs, though sometimes it does, maybe since it was tagged that way in MP3Tag in the first place, I'm not sure).
If I tag in MP3Tag the disc number in the format disc_number/total_discs Picard sees the total_discs value.
Here is when it gets really weird. I use a script in MP3Tag to rename the files, so if total discs is greater than 1 to rename the file (disc number)(track number) - title (e.g. "112 - song_name.mp3"), and if there's only one disc to rename it as (track number) - title (e.g. "12 - song_name.mp3")
Having Picard set total discs value doesn't guaranty it will work, having it tagged in MP3Tag in the format disc_number/total_discs doesn't guaranty success as well.
Sometimes it works (in MP3Tag) even if the disc number tag is in the format disc_number without total_discs, sometimes it fails when it is tagged in the format disc_number/total_discs.
I couldn't find any logic there.
MP3Tag doesn't have TPOS.
I don't know how to rename in Picard in that way, esp. since it is automatic so in case of various artists album I will want to add the artist name, but I'm not familiar with the script in Picard.
Hope I was clear