There is already some sense implemented.
If there is the value given "3/21" in the tag-field TRACK, then Mp3tag uses this value, why not?
So you can copy or append or prepend or insert this value to some other tag-field or put it into a text file by a Mp3tag report file.
When using the value "3/21" to create a file name or folder name, Mp3tag automatically removes such characters, which are forbidden to use for names in the file system, so the value will be changed to "321".
In the given limits you can control the validating to your own needs by using the function
See Mp3tag manual.
See also ...
[F] $validate() broken?
On the other hand you can provide the tag-field TRACK without the total tracks part.
Simply use two separate tag-fields TRACK=3 ... and ... TOTALTRACKS=21.
Use function $num(,) to format the track number ...
Use function $num(,) to format the number of total tracks ...
If needed, then build a format string of both components ...
With TRACK having the string '3/21'... you can do ...
$replace(%TRACK%,'/',' of ')
... which gives the result: '3 of 21'