If you want to use an Excel workbook as the data source, simply create a column of numbers, with a header in row 1. The number format in the mailmerge output is controlled by the application of a numeric picture switch, for which see
Mailmerge Number & Currency Formatting in the
Mailmerge Tips and Tricks 'Sticky' thread at the top of this forum:
https://www.msofficeforums.com/mail-...ps-tricks.html