[...to count discs...] we need an extra storage place to hold a user defined value while
an action runs against the selected tracks
Since then, Mp3tag as acquired actions for Export and Import. So perhaps a solution is: an Export action counts the discs and writes the results to a text file, and then a Import Text File action does a lookup to write them into the tags.
Though this would not work before the fix to the bug at /t/5799/1 .