iBooks – 223 e-books
Kindle for iPhone/iPad/Mac – 191 e-books
Nook for iPhone/iPad – 9 e-books
Google Play – 11 e-books (all public domain)
Kobo – 3 e-books (all public domain)
Bluefire Reader – 3 e-books (2 Filipino, 1 Portuguese bought)
Elefant (Romanian) – 3 e-books
Classic Books – 3 e-books (all public domain)
Stanza – 7 e-books (all public domain)
Total – 453 e-books
More and more, I've been favoring Apple's native iBooks e-reader, as the screen is brighter and its pagination system (which includes the remaining e-pages in chapters) makes for a more enjoyable e-reading experience.
Now I haven't done a count of my paper books for a couple of years now, but with all of the recent purges and boxing up, I would guess that I probably have somewhere in the neighborhood of 1700-1800 print volumes, with dozens arriving and leaving each month. I suspect that in the coming years, the print and e-book numbers will achieve a rough balance, as it is sometimes convenient to read a pre-1923 e-book on the iPad's screen for $0.00 paid and there have been quite a few of these public domain e-books (many, but not all, from Project Gutenberg) added to the reading e-queue for the next few months.
What about you? If you read e-books, do you have a rough idea about how many you have on your e-reader(s)?