You could certainly number individual copies, but anyone who was going to
fradulently use your document would have no difficulty changing (or
removing) whatever numbering scheme you apply. If you allow someone to see
your document you lose any control you had over its content.

While I agree completely with what Graham says about security, if you do
want to put a number on the individual copies of a document that you
produce, see the article "Sequentially numbering multiple copies of single
document using a macro" at:

http://www.word.mvps.org/FAQs/MacrosVBA/NumberCopiesOf1Doc.htm

or, "Sequentially numbering multiple copies of single document using a
mailmerge" at:

http://www.word.mvps.org/FAQs/MailMerge/NumberCopiesOfDocMMerge.htm

To thwart you however, all someone has to do is to reproduce your document
without the number.  Then you would not be able to trace the source of the
unauthorized copies.

You can make it harder by putting the number in the header, or footer, or as
a watermark behind the text and protecting the document with a password, but
if someone is determined enought, they can always just re-type the document.