Just what you want to achieve is a little bit garbled.

With a formletter type mail merge, were information generated for each
recipient is usually sent to only one person, it is appropriate that the
first page of the information for each recipient is assigned page number 1.
If you are executing the merge to a new document, to get the total number of
pages for each person to be displayed, you need to use the SECTIONPAGES
field in the mail merge main document in place of the NUMPAGES field.  That
field will return a count of the number of pages in each Section of the
document, as a new Section is created for each record in the data source.

If that does not give you what you want, please try and explain more clearly
what it is.