2 page number in one page

You can add a separate text frame to the Master Page(s), with a Flow letter different than "A", perhaps "Flow P".

It could be 1 line high.
The text in it would just be an auto-numbered paragraph, perhaps of stylename FilePage, with an AN Definition of <n+>, with Next Para set to FilePage. It may not be necessary to have a separate <n=1> variant.

As content pages are created/added, just hit [Return] in the little text frame to have a number appear.

This content can be visible or not in the final work, using Color Views or Conditional Text. The number can be Cross-Referenced as needed for in-line mention or hyperptext.

It's a bit hacky, but seems to work.

Hitting [Return] in that text frame can crate new empty pages. Where unwanted, just backspace them out & save.

I am not sure I understand the question properly. You can ofcourse have a standard page number going from 1 to the very end. And  you can control how this number is expressed using the books numbering features, making it for instance use roman numbering or characters depending the chapter. But what I need to understand better is how you imagine "relevant section page number." works? What would it look like?