One! But you can merge to three general document types :
Form letter - one record per page
Directory/catalog - multiple records per page
Mailing Label - multiple records in a table.
and you can use a vast variety of data formats.

There are four types of mal merge main document, five if you include "not a
mail merge main document". The types are

wdNotAMergeDocument    = -1
wdFormLetters = 0
wdMailingLabels = 1
wdEnvelopes = 2
wdCatalog = 3

In Word 2002 "Catalog" has been renamed as "Directory" and the folowing new
constants are defined:

wdDirectory = 3
wdEMail = 4
wdFax = 5

There are potentially 4 types of mail merge destination -
 mail to printer
 mail to new document
 mail to e-mail
 mail to fax

--
Peter Jamieson - Word MVP
Word MVP web site http://word.mvps.org/