This is a many to one mail merge -
See
http://www.gmayor.com/ManyToOne.htm (sort the data on the number field and use that as the key field).
Paul Edstein has prepared a tutorial employing a different method based on the Microsoft support link (below), with working field codes and a sample Excel data source which you can also download from my website
http://www.gmayor.com/Zips/Catalogue%20Mailmerge.zip
Microsoft's approach is at "How to use mail merge to create a list sorted by category" -
http://support.microsoft.com/?kbid=294686