Tricky question, really.
Since I have the (cleaned-up) Black Mask Collection on DVD, that puts me in the 25,000+ bracket.
Otherwise, I'd be in the 10-25,000 bracket, thanks to the 14,000 Gutenberg titles I d/'ed, "prettified" and converted to mobi and MSReader format ages ago. (Two whole weeks I had the PC running those batch files.)
Plus, a few hundred classic literary ebooks I picked up from university ebook repositories in ages past (University of Virginia, etc). Had to keep the PDA fed...
Other than that, I have 13 years of Webscriptions and the BAEN promo CDs and a good chunk of the Free Library, say 1000 titles, a couple hundred Fictionwise legacy buys, and a couple hundred Kindle titles. Plus a few dozen titles from Powell's and Amazon from the first age of commercial ebooks. (All MSReader format, post-processed, of course.)
So, classics aside, I'm in the more reasonable 500-2000 bracket for purchased ebooks.