I never ever knew you could do that after being on this Forum for all these years
If you go to User CP, choose Edit Options (it's the 4th on the left side). You see Thread Display Options then number of posts to show per a page part, I think you have 'use forum default'. You can choose there whatever you want, 5,10, 20, 30 or 40 post per page. If you think. You should know that what is the easier for you.