Hello,
Internally, DOCX documents work with sections, not pages. A section may fill one or more pages or even less than a page (several sections may fill one page). How many pages have a section is unknown until the DOCX is opened using a DOCX reader (in some cases, distinct DOCX readers may show more or less pages for the same DOCX depending on the contents).
Unfortunately, a DOCX doesn't include a specific tag or content to set a page, only sections. You can do page breaks (addBreak method) and section breaks (addSection method) if needed to separate contents; the previous methods add OOXML tags that can be queried, added, changed, removed...
Regards.