He actually answered this in another thread -
Is there a way to change to number of posts per page? I can't seem to find that. But, the page count on some threads I'm quite familiar with just jumped a lot.
No there isn't. This is because it is now possible to link to specific pages of threads. eg.
http://forums.wdwmagic.com/threads/feedback-on-new-style.838970/page-4
Allowing a user to change the number of posts per page would mean that a link to a page could result in a different set of posts appearing for different people.