I read on nearly all of my devices (and on a couple of not-bought-for-reading devices as well), so I think I have just enough.
The only one I don't read is the one with the dead battery -- my PRS-505 -- and the only reason it is still not charged is because its content needed to be updated and I don't want to do that. I really like just getting my books out of the air instead of having manage them at my computer.
Current books:
* Lord of the Rings (NOOK HD+, Kobo app)
* The Call of Zulina (iPhone, Kindle app)
* River in the Sky (iPad, Sony app; would be PRS-505 if it was charged)
* Sense and Sensibility (T1, downloaded from MobileRead)
I alternate books between my Kindle and my T1; that is, when I've finished a book on the T1, I read a book on the Kindle. The NOOK HD+ was supposed to be used as an Android tablet, bought after B&N allowed the Play store on it, but so far I have read one NOOK book and downloaded the Kobo app so that I could read LOTR.
I could read all of those books on my iPad -- which I never intended to use for reading; I bought it for sharing photos and watching videos -- in the appropriate apps, but I guess I'm old-fashioned where I like for each book I am reading to be physically distinguishable from any other book that I am reading. I don't want to be going between multiple books on a single device.