I know this is a tad bit old however I happened to stumble on this post while I was looking for a way to increase both the page to 100 posts and the parent page to show all 100 posts as well. The solution above adding to the functions page did not work for me even upon changing it to the proper code.
I did however find the same solution as jameswordpress did but there are multiple lines.
to edit so that both the parent forum and the selected forum page to show more than 50 forums.
To do this edit template.php located in:
/bbpress/includes/forums/template.php
Change line 135:
'posts_per_page' => get_option( '_bbp_forums_per_page', 50 ),
to:
'posts_per_page' => get_option( '_bbp_forums_per_page', 100 ),
———-
Also changing line 708 should effect the parent page (at least it did for myself):
'posts_per_page' => get_option( '_bbp_forums_per_page', 50 ),
to:
'posts_per_page' => get_option( '_bbp_forums_per_page', 100 ),
You can change the value of 50 to whatever you desire.
