I think your arithmetic expression is a fancy way to say "2".
If you want to count the number of documents meeting your criteria inside the file cabinet, you need to look it up in the database in your "assign data" step, however, this only works if your expected result is 100 or less.
f you expect more than 100 entries, you may need to use a webservice which searches for the number of documents and returns the number of results or rethink if this is the only way to reach your goal.
Hope this helps.
Greetings from Germany,
Simon H. Hellmann
DocuWare System Consultant