See the "Individual Merge Letters" item on fellow MVP Graham Mayor's website
at:

http://www.gmayor.com/individual_merge_letters.htm

If you are using Word XP or later, the "Add-in to Merge Letters to Separate
Files" that I have written and that can be downloaded from that site will
allow you to create each letter as a separate file with a filename taken
from a field in the data source with a minimum of fuss.

When you create individual documents in one of those ways, a NUMPAGES field
should then show the correct total of the pages in each document.

Otherwise, you can use a SECTIONPAGES field, which will display the number
of pages in each Section of the Document (When a formletter type document is
merged to a new document, the letter for each record is contained in a
separate Section.)