Scroll down to the Boards & Threads section. Change the numbers for Threads Per Page and for Posts Per Page to whatever amount you desire.
Adjusting the amount of threads or posts that are displayed per page
To adjust the amount of threads displayed per page in a board or to adjust the amount of posts shown per page in a thread, open the admin panel and select Settings, then select Forum Settings on the following menu.
Scroll down to the Boards & Threads section. Change the numbers for Threads Per Page and for Posts Per Page to whatever amount you desire.
Scroll down to the Boards & Threads section. Change the numbers for Threads Per Page and for Posts Per Page to whatever amount you desire.