That's something covered in my own
Microsoft Word Catalogue/Directory Mailmerge Tutorial, available at:
http://windowssecrets.com/forums/sho...merge-Tutorial
or:
http://www.gmayor.com/Zips/Catalogue%20Mailmerge.zip
In particular, see the topic
Calculating Group and Sub-Group Totals. Do read the tutorial before trying to use the mailmerge document included with it.