A page for posts is always in a topic, and always used to be 15 posts long, no matter how long or short the posts are. Pages for topics are always in forums and usually contain 20 pages, unless special viewing options are used.
Before The 2012 Update, Keighlian posted an easier way to go through pages, by using the URL and typing the page number in for shortcuts. After the 2012 Update, users can choose in their settings how many posts to have per page, and how many topics to have per page.
The topic with the most pages is Finding Friendships, with over 31,427 pages.