You can number the paragraphs in your books by adding the following code to your CSS:
Please note that:
a) This method may or may not have been exhaustively tested for use in all formats and themes, and
b) Pressbooks cannot offer comprehensive technical support for customization to users' CSS.
For more information on how to modify your CSS in Pressbooks, check out our user guide: https://guide.pressbooks.com/chapter/customizing-your-exports-with-custom-styles/