The manual says: %_total% Total number of tracks from xx/xx track-number field
Before you can use the content form the variable %_total%, for example for renaming purposes, you have to make sure, that the tagfield TRACK contains a data structure as of "tracknumber/totaltracknumber", for example "08/15".
In this case the variable %_total% will be able to return the value "15", ...
even when only one file has been selected.
At first you can prepare a tagfield TOTALTRACKS.
Select all files from a logical set of files (folder, album or such), for example 5 files.
Then apply ...
Action: Format value
Tagfield: TOTALTRACKS
Formatstring: %_total_files%
All files will get the tagfield TOTALTRACKS filled with the number "5".
Then apply ...
Action: Format value
Tagfield: TRACK
Formatstring: $if(%TOTALTRACKS%,%TRACK%'/'%TOTALTRACKS%,%TRACK%)
Now all the tagfields TRACK should contain a text string, for each ...
"1/5", "2/5", "3/5", "4/5", "5/5".
Now you can use the variable %_total% for renaming purposes.
If the tagfield TOTALTRACKS still exists, then you may use the value variable %TOTALTRACKS% for the same purpose.
See also ...
What is the correct format for numbering tracks?
Note: The media file type MP4 can hold only one numerical value in its track tagfield.
For MP4 Mp3tag delivers the variable %_total% as empty.
DD.20150717.1254.CEST, DD.20150722.1955.CEST