[Quoth]

You don't know what number of pixels wide & high a page is.
You don't know ereader/app ppi.
You don't know physical screen size /ratio
(Media queries only work on some apps and ereaders and platforms).

Epubs that work on any size screen reflow. A new file causes a new page.
Once the page is full, the there is a new page.

The user can change margins, relative line spacing (if there is no line-spacing) and relative font size, so there is no fixed number of pages for two people using the same ereader or same app on same tablet/phone.

Never include line-spacing. It will be automatically suit the font and size, and if not set, the user can change it.

It's only relevant for PDFs and so-called "fixed layout"/"Print replica" to decide on page content in advance, and they need a large screen and are not true ebooks.

Some ereaders will not display if a file is too large.

We test on 4.3" to 10.3" screens for any new kind of content.

Do not be concerned about I/O. In general the more files the better, and a new file starts a new page, so novels at least have a file per chapter. Comics might have a file per layout, so in some cases are one image per file.