FYI: Each book's publisher decides how many licenses each ebook purchase comes with. Generally, they come with at least three to five licenses, in my experience. That means you can download a particular Kindle ebook onto three to five devices, no matter what kind of device, on whatever platforms Kindle supports, as long as you use the same account. Once you reach the maximum number of licenses, you have to either buy a new copy if you want to download the ebook yet again, or you can talk to Amazon and see whether they'll reset your licenses. Audiobooks from Amazon and Audible (which Amazon owns) work similarly.
So far, I've had Amazon reset an ebook or two, and they've obliged, but I don't know what they'd do if I suddenly needed my entire library reset. Audible also has obliged me when I've hit the max number of copies for particular audiobooks.
For some people, this might take longer to become a problem, if they don't own many devices and/or they go many years without upgrading devices. But longer term, we'll all presumably encounter license limits on ebooks (and audiobooks) unless we don't care to access them over the years. For people who don't reread books, it's not a prob.
