After you switched on “Show Track Numbers” in Music library settings, you must make a new playlist because setting not changes playlists made before new setting. Possibly to see track numbers in library browser you might need to re-scan library, but I’m not sure of this. Also if you have no track number in tag or no tag it will not work.
I had looked in the appearance section, but not in the music library section. For anyone else, In the ‘My Music’ page of the settings is a section called ‘Music Library Settings’ (second last section). Here you will find the ‘Show Track Numbers’ option.
It does seem an odd place for it - its not a music library setting at all, but an interface display option, but at least I was able to find it…