1. Amazon's Kindle books are already over 300,000... they add 500+ books per day typically.
2. No one else has even that many. Note this is not counting the millions of public domain books out there for free that will work on almost any device. There are very few books that are available for another reader that are NOT also available on Kindle, but quite a few books that are available ONLY on Kindle and not elsewhere.
3. Yes, Amazon has 26 million (more now) print books listed. But remember, millions (maybe even more than 50%) of those books are simply placeholders for books that are not currently in print; they allow you to buy and sell used copies. There's no easy way to tell exactly how many books are currently in print via Amazon. The nice thing about ebooks is that they are always in print.
4. So the real question is, not whether you can get an ebook for a book that's out of print physically (though that is a nice advantage of ebooks, and you often can), but whether or not you can buy new ebooks for books in print. And the answer to that is generally yes, though there are still quite a few noticeable gaps. The industry is changing; 5-10 years from now every newly released book will have an ebook to go with it.