Try this:
Near the top of this page there's UserCP
Click on it.
Under Settings & Options click on Edit Options
Down that page a bit is Thread Display Options
Set Number of Posts to Show Per Page to at least 20.

Oh yeah - You have to scroll down to the bottom of that page to make the changes actually change.

BTW - You can open the forum in another tab and keep this on the screen while you go through the menus.

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In dog years I'm dead.