Under " USER CP" link (left side gold toolbar), select "Edit Options"
Scroll to this box:
<TABLE class=tborder cellSpacing=1 cellPadding=6 width="100%" align=center border=0><THEAD><TR><TD class=thead>Thread Display Options </TD></TR></THEAD><TBODY id=collapseobj_uopt_thrd><TR><TD class=panelsurround align=middle>
<FIELDSET class=fieldset><LEGEND>Visible Post Elements</LEGEND><TABLE cellSpacing=3 cellPadding=0 width="100%" border=0><TBODY><TR><TD>You have the option to show or hide various elements of messages, which may be of use to users on slow internet connections, or who want to remove extraneous clutter from posts.</TD></TR><TR><TD><LABEL for=cb_showsignatures><INPUT id=cb_showsignatures type=checkbox CHECKED value=1 name=options[showsignatures]>Show Signatures</LABEL><INPUT type=hidden value=1 name=set_options[showsignatures]>
<LABEL for=cb_showavatars><INPUT id=cb_showavatars type=checkbox CHECKED value=1 name=options[showavatars]>Show Avatars</LABEL><INPUT type=hidden value=1 name=set_options[showavatars]>
<LABEL for=cb_showimages><INPUT id=cb_showimages type=checkbox CHECKED value=1 name=options[showimages]>Show Images (including attached images and images in [IMG] code)</LABEL><INPUT type=hidden value=1 name=set_options[showimages]>
</TD></TR></TBODY></TABLE></FIELDSET> <FIELDSET class=fieldset><LEGEND><LABEL for=sel_threadedmode>Thread Display Mode</LABEL></LEGEND><TABLE cellSpacing=3 cellPadding=0 width="100%" border=0><TBODY><TR><TD>Here you can choose the display mode for threads. For a full explanation of the modes, please view
this help page.</TD></TR><TR><TD><SELECT id=sel_threadedmode name=threadedmode> <OPTION value=0 selected>Linear - Oldest First</OPTION> <OPTION value=3>Linear - Newest First</OPTION></SELECT> <LABEL for=sel_threadedmode>Thread Display Mode:</LABEL> </TD></TR></TBODY></TABLE></FIELDSET> <FIELDSET class=fieldset><LEGEND><LABEL for=sel_umaxposts>Number of Posts to Show Per Page</LABEL></LEGEND><TABLE cellSpacing=3 cellPadding=0 width="100%" border=0><TBODY><TR><TD>Use this option to set the number of posts to show in a thread before splitting the display into multiple pages.</TD></TR><TR><TD><SELECT id=sel_umaxposts name=umaxposts> <OPTION value=-1>Use Forum Default</OPTION> <OPTION class="" value="" selected>Show Posts Per Page</OPTION></SELECT> <LABEL for=sel_umaxposts>Number of Posts to Show Per Page:</LABEL> </TD></TR></TBODY></TABLE></FIELDSET> <FIELDSET class=fieldset><LEGEND><LABEL for=sel_prunedays>Default Thread Age Cut Off</LABEL></LEGEND><TABLE cellSpacing=3 cellPadding=0 width="100%" border=0><TBODY><TR><TD>You may specify a time period from which to display threads. Threads older than the age you specify will not be shown.</TD></TR><TR><TD><SELECT id=sel_prunedays name=prunedays> <OPTION value=0 selected>Use Forum Default</OPTION> <OPTION value=1>Show threads from last day</OPTION> <OPTION value=2>Show threads from last 2 days</OPTION> <OPTION value=7>Show threads from last week</OPTION> <OPTION value=10>Show threads from last 10 days</OPTION> <OPTION value=14>Show threads from last 2 weeks</OPTION> <OPTION value=30>Show threads from last month</OPTION> <OPTION value=45>Show threads from last 45 days</OPTION> <OPTION value=60>Show threads from last 2 months</OPTION> <OPTION value=75>Show threads from last 75 days</OPTION> <OPTION value=100>Show threads from last 100 days</OPTION> <OPTION value=365>Show threads from last year</OPTION> <OPTION value=-1>Show all threads</OPTION></SELECT> <LABEL for=sel_prunedays>Default Thread Age Cut Off:</LABEL> </TD></TR></TBODY></TABLE></FIELDSET>
</TD></TR></TBODY></TABLE>
Instead "Use Forum Default", select show posts per page.....
Hope that helps.