I just had to do this while working on a new template. I am going to paste my entire code here, even though the formatting will differ from Wistie. Hopefully it will demonstrate to you what you need to do. You would basically just have to change 10 to 5.
Code:
{if count($related_story) neq 0}
<div class="tab-pane fade" id="related">
<h3>{#PLIGG_Visual_Story_RelatedStory#}</h3>
{checkActionsTpl location="tpl_pligg_story_related_start"}
<ol>
{* The next line checks how many related stories there are. If there are fewer than 10 it will set the return loop to a smaller number. *}
{if count($related_story) > 10}{assign var="related_count" value="10"}{else}{assign var="related_count" value=$related_story}{/if}
{section name=nr loop=$related_count}
<li><a href = "{$related_story[nr].url}">{$related_story[nr].link_title}</a></li>
{/section}
</ol>
{checkActionsTpl location="tpl_pligg_story_related_end"}
</div>
{/if}