I tried to see if Plone board allows us to increase the number of posts per page but it didn't. Also you can't see all posts of a thread in a page.
What you can do is use the "Print this" link at the bottom of the thread page to get a PDF file for your reading pleasure. You get the whole posts in a thread in a single pdf. I do this all the time to read threads of my interest and important at leisure and at office.