Word's NUMWORDS field has no means of defining a range for the count. Indeed, from the first refresh onwards, it includes itself in the count. What you can do is use a formula field coded along the lines of:
{={NUMWORDS}-n}
where n is the count of words on your cover page. To create such a field, press Ctrl-F9 twice to create a nested pair of field braces (i.e. {{}}), then add the text to provide the above construction, replacing the 'n' with the cover page word count +1. Press F9 to update when you're done.

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Cheers,
Paul Edstein
[Fmr MS MVP - Word]