The "bottom line":
You are able to redownload your books an unlimited number of times to any specific device.
Any one time the books can be on a finite number of devices. In most cases that means you can have the same book on six different devices.
Unfortunately the publishers decide how many licenses, that is devices, a book can be on at any one time. While most of the time that will be five or six different devices there will be times when it's only one device.
At the present time there is no way to know how many devices can be licensed prior to buying the book. [Emphasis added.]
I guess we shouldn't be surprised.
Cross-posted at Against Monopoly.