I can't upgrade mine, it's in a production environment. Is the dokuwiki.org site using Lazy Sunday? If so, the bug is present there as well. It could be tested on any site just by using the URL feed.php?type=rss2&num=5
Works fine for me. dokuwiki.org uses feedburner which probably breaks this feature. Note that the limiting only works for recent changes feeds.
Works here also ("Anteater" and DW-2009-02-14).
As reported by several people the num parameter isn't broken and dokuwiki.org is a special case because of the use of feedburner.