Add a new column to your view by right-clicking the view header in the file info pane.
Or, use the Tag section in the Action window. Add Disc # as a field to be shown if it is not already there.
Select all the tracks from one disc at a time, add the disc #. When this is done, select all the discs in the multi-disc album and ensure the title is what you want (and does not say "Varies").