The simplest way of doing this would be to create an Excel workbook to use as a mailmerge data source. In the Excel workbook, you might have:
A1 'Bag No.'
A2 your starting #
A3 the forumula: =A$2+INT(ROW()/2)-1
and copy the formula in A3 down as far as needed. Save the workbook then, in Word, set the document up for a mailmerge using the Excel workbook as the data source and insert the «Bag_No» mergefield. If you then 'Finish' the merge, the merged output document will contain all your numbered pallet placards.

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Cheers,
Paul Edstein
[Fmr MS MVP - Word]