Amazon Kindle e-books aren't identical to printed books. Pages in an e-book don't necessarily correspond to pages in a printed book. Amazon added a feature in the Kindle 3.1 software that can show the exact page numbers that correspond to the location you are in within the e-book. You can use the page numbers to reference page numbers in the physical book. Only certain Kindle e-books contain support for page numbers.
Open an e-book on your Kindle, and navigate to the location of which you want to see the page number.
Press the "Menu" button on your Kindle.
Read the page number from the bottom of the Kindle's screen, if page numbers are available for the book you're reading. Page numbers are displayed as "Page X of [Total]" to the left of the "Location" label.