Using Mp3tag list view, in general you may visually detect identical field content by sorting the specific column, so identical values are arranged together.
If you want to add a running number to a field content, then select the related files, ...
and apply an user defined action, which automates the adding of a running number, ...
otherwise do the changes manually.
If you are fit enough in building up and programming a Mp3tag export script, then you may automate the process of numbering, e.g. by creating a CSV text file, which contains the wanted changed values. Afterwards the values from the textfile can be imported into the tags within the related files.
The placeholder %_counter% delivers a running number, starting at number 1.