Track Number isn't a TV Tag (Much)
I have to admit, it's hard to keep in mind what tags from iTunes are still available for which media in the Music and TV apps.
Case in point: the track number tag. A Track Number is the index of a track in its album play order. Every school child knows that (well, every school child familiar with "album"). Strictly speaking, it is a music-related tag. But because iTunes handled music and video tracks, ALL tracks had a track number tag.
The TV app apparently does not care to have much truck with the Track Number tag; it does not appear in a track's Show Info panel. But it can appear as a column in List View. And it is still accessible by AppleScript.
I heard from a Correspondent who was concerned about his pristine TV Show track numbering largely being obscured by the TV app. I suppose you could: 1) Put the list of TV Shows in List View, 2) Show the Track Number column and 3) Command-I each track and manually enter the Track Number into the episode field.
Or, run this script on a selection of TV tracks to copy the track number to the episode number: