Hi Kenneth,
search-total will tell you the number of documents matching the search. You can't tell the API to just return that number, and not the reference to the documents matching the search, but if you don't really care about them, you could in the query say <page-size>1</page-size><page-number>1</page-number>, and you'll have at most 1 document returned.
Alex
--
Follow Orbeon on Twitter: @orbeon
Follow me on Twitter: @avernet