How do I know what tag name is used in VORBIS comments? The help page doesn't have a column for Vorbis

With Vorbis Comments the field name is directly used inside the tag. There are three exceptions:

• TRACK is mapped to TRACKNUMBER
• YEAR is mapped to DATE
• PUBLISHER is mapped to ORGANIZATION

These built-in mappings try to align the user's input with the current VorbisComment recommendation for field names.

So, if I have understood correctly, the Mp3tag "comment" is mapped to the VorbisComment field name "COMMENT". What VorbisComment field name is the Mp3tag "total number of tracks" mapped to?

I ask because I am helping Xiph (Vorbis) update their recommendations for field names, and I want to know what names are already being used in the wild.

There is no fixed field name for that. Mp3tag is very flexible in this regard, you can use anything what is most compatible with the other programs/devices you're using.