There is no page count, only word count. You can change the font size on an e-reader which makes page numbers irrelevant. The Kindle has a percentage read bar on the bottom.
ADE DOES have a page count. It's based on 1024 bytes of the compressed ePub. When you move from one flow to another, that also changes the page number. The page number in ADE is not based on font size or screen size. Changing the font size does not change the page number. The page number for a given book is constant. So when you see a page number of say 257 for an eBook, it's the same regardless of the font size.
The ADE page number does not correspond to the printed editions page number. Even different format print editions (hardcover vs paperback, trade vs mmpb) do not correspond to each other.
Given that ADE is the standard for reading ePub, the page numbering used is the standard for ePub.