The "report" did not say "18,000 al Qaeda fighters in Iraqi."
The International Institute for Strategic Studies claims that
there are up to approximately 18,000 in the whole world.
But someone in another thread provided info on The International Institute for Strategic Studies ("Does the IISS have a political agenda?") that you should all have a look at.
http://www.freerepublic.com/focus/news/1142187/posts?page=9#9
Cool, and more plausible, which does not mean it is plausible. How the hell did this outfit come up with such a number?