Mp3tag already supports the quasi-standard fields TRACKTOTAL, DISCNUMBER, and DISCTOTAL. Since VorbisComments allow for arbitrary field names, some users — depending on the other software and hardware they're using — choose TOTALTRACKS and TOTALDISCS.
If you want to make use of those fields from within Mp3tag, you can customize the Tag Panel and the File List accordingly.
See also Tag Mapping → Counts & Indexes at the Hydrogenaudio Knowledgebase and Field Names at the Xiph Wiki for details on the different fields used out in the wild.