Go to your "User CP" in the upper-left corner of the forum.
Under "Your Control Panel", click "Edit Options" (under Settings & Options).
Page down until you get to "Thread Display Options".
Under "Number of Posts to Show Per Page", select how many posts you want to be displayed at a time (I believe 30 is usually the default, although I have mine set to 60 posts per page).
Page down to the bottom and click "Save Changes".
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