The default setting is 15 posts per page. If you set it to 40, it should adjust immediately, but you may have to log out and log back in again to see the change. However, I just set mine to 40 to test it, and whew did the page take a long time to load! The long signatures will also affect this. 15 usually loads pretty quickly. People with really slow dialup accounts may want to change it to 5 or 10.
The number of threads per page (in the forum thread list) is set to 20 and cannot be changed.
Hope that helps