This will depend upon the type of content your book contains, the eBook formats that it is converted into, and the type of device/software that the reader is using to view your eBook, along with their own personal preferences and settings.
Most eBook reading devices, such as the iPad and Kindle, allow the reader to choose which font-faces and font-sizes they want displayed, which in turn can change the amount of content that is visible on a single “page” (screen). This means that with standard formats eBooks, i.e. Mobi and ePUB formats, that two people reading the same book on the same type of device may see quite different displays based on their own personal settings.
When we produce standard format eBooks, we ensure that your eBook is be formatted to perform across all the major reading devices and software.
With Fixed-layout eBooks, the content of each page is fixed so the reader cannot control the text size or font used. This means that Fixed-layout eBooks can be created to closely replicate the layout of more complex titles or heavily illustrated books.