How do I limit the number of pages that a libre word document has?
There is no limiting option for number of pages. Pages aren’t primary items in Writer; user doesn’t create pages, user creates content with formatting (including marks telling that this content must start on a new page, optionally with specific page style), and Writer internally, on-the-fly, creates necessary pages for you to hold all your content.
If you need not some pages, this means that actually you don’t need the content on those pages (it might be even empty paragraphs, or zero-length graphics). So, you should delete the content of those pages, and the pages will be gone automatically.