Well, here’s a surprise—eBooks don’t have page numbers.
Because text reflows there are no page numbers in ebooks. As you can see, it would be silly to put page numbers when the pages or screens change all the time. A word could be on “page 13” one minute, and then with a user change of font size, on “page 15" the next.
In fact, an eBook’s “pages” are actually an illusion, created by the device. In reality, an eBook is one very long webpage (or a series of very long webpages), and the view of the “pages” changes as each device opens it, and as each human reader chooses his font size and, in some instances, the font. Some devices, like the Nook, create "faux" or fake page numbers, to give the human reading it a sense similar to that of a print book. For example, in "Frumpy to Fabulous: Flaunting It," shown below, you can see that the Nook device has created a fake page number in the upper-right-hand corner, along with a "running header" with the title name. These are both functions of the device, and are not actually present in the e-book itself.
The Kindle devices use "Locations," rather than the faux page numbers, and they serve much the same purpose; to give the reader a basis from which to judge his or her progress through the book.
Some e-reading software also creates fake page numbers. One program in particular, Adobe Digital Editions, which you'll likely use in your review of your ePUB ebook, creates a "page number" every 1,000 characters, that you can easily use for reference while proofing your book. You can see an example of this in the yellow-highlighted spot below--Adobe Digital Editions ("ADE") creates a fake page number, that does not exist in the actual ebook. While displayed on the page in ADE, it is not displayed this way on devices like the Nook, as you can see in the first example. (n.b.: in 2015, when this article is being updated, ADE displays page numbers that it creates at the bottom of the page; this image, below, is from the 2010 version of ADE.)