For those three or so in this thread who have asked about number of posts per page and received no answer…this bugged me so I went looking for an answer.
Apparently, this appears to be the one foible of Xenforo. In order for the forum software links to function in a user friendly/SEO form there is no stock control for the user to change the number of posts per page. Xenforo links to the post on the specific page and not the post specifically (although you get the link from the post number) so allowing users to determine their own posts per page would throw links off. Apparently Xenforo devs are quite defensive about this deficiency too. So, the new wonderful forum software can't link to individual posts but the old software can.
The administrator of a site has this option, but it's system wide, not specific to any user. The default is 20, apparently we have been allowed 25.
Apparently, as well, there is a third party addon or plugin available that allows this, but presumably that has not or will not be installed here.