Put the cursor at the end of the document where you want the count to
appear. Go to the Insert > Field dialog and select the NumWords item. That
will insert a field that displays the same number. (It's just the number. If
you want it to say something like "5000 words" you'll have to add the space
and the "words" manually.)
The field doesn't update automatically as you edit the document. Either
select it and press F9, or right-click it and select Update Field, or go to
Print Preview and back.