Somewhere between 700-900... I really haven't had time to COUNT everything, but to put it in perspective, I have almost everything by Rush, Dream Theater, Porcupine Tree, Transatlantic, Spock's Beard, The Flower Kings, Genesis, Yes, The Beatles, and many other bands' full studio discographies... and many of the aforementioned bands' non-official stuff I own too, so between CDs (studio, live, etc), and DVDs (concerts, documentaries, etc.), it adds up.
Let's put it this way - I've spent too much of my income in the last decade on music, and I (more often than not) prefer to buy the physical product than a digital download. You know those Caselogic 92/100 CD case wallets? I own 10 of those and they're all full of CDs and DVDs... so that's somewhere around 800...
Maybe someday I'll actually count everything out and set up some sort of Office document to keep track of it all!
-Marc.