This template edit hides the member count from guests.
Open forum_list and find:
<dl class="memberCount"><dt>{xen:phrase members_count}:</dt>
<dd>{xen:number $boardTotals.users}</dd></dl>
Add this before:
<xen:if is="{$visitor.user_id}">
and this after:
<xen:else />
</xen:if>