Do you want it in all forums or just one? In admin panel
Look & Feel >
Manage Skin Sets & Templates >
Manage Templates >
Forum View > forumIndexTemplate
find this code
<if test="showtopics:|:$forum_data['sub_can_post']">
<div class='topic_controls clearfix'>
{$forum_data['SHOW_PAGES']}
<ul class='topic_buttons'>
<if test="usercanpost:|:$forum_data['_user_can_post'] && $this->memberData['posts']>20">
<li><a href='{parse url="module=post&section=post&do=new_post&f={$forum_data['id']}" base="publicWithApp"}' title='{$this->lang->words['topic_start']}' accesskey='s'>{$this->lang->words['topic_start']}</a></li>
<else />
<li class='disabled'><span><if test="isGuestPostTopicTop:|: ! $this->memberData['member_id']">{$this->lang->words['forum_no_start_topic_guest']}<else />{$this->lang->words['forum_no_start_topic']}</if></span></li>
</if>
change to this for all forums and 20 posts . You can make the 20 any number you want 0 or 1 or ??.
<if test="showtopics:|:$forum_data['sub_can_post']">
<div class='topic_controls clearfix'>
{$forum_data['SHOW_PAGES']}
<ul class='topic_buttons'>
<if test="usercanpost:|:$forum_data['_user_can_post']">
<li><a href='{parse url="module=post&section=post&do=new_post&f={$forum_data['id']}" base="publicWithApp"}' title='{$this->lang->words['topic_start']}' accesskey='s'>{$this->lang->words['topic_start']}</a></li>
<else />
<li class='disabled'><span><if test="isGuestPostTopicTop:|: ! $this->memberData['member_id']">{$this->lang->words['forum_no_start_topic_guest']}<else />{$this->lang->words['forum_no_start_topic']}</if></span></li>
</if>
change to this for a single forum or a selection of forums to use single forum change array(1,3) to array(1) to add more just add a , then the added forum id like this array(1,3,21)
<if test="showtopics:|:$forum_data['sub_can_post']">
<div class='topic_controls clearfix'>
{$forum_data['SHOW_PAGES']}
<ul class='topic_buttons'>
<if test="usercanpost:|:$forum_data['_user_can_post'] && in_array($forum_data['id'],array(1,3)) && $this->memberData['posts']>20">
<li><a href='{parse url="module=post&section=post&do=new_post&f={$forum_data['id']}" base="publicWithApp"}' title='{$this->lang->words['topic_start']}' accesskey='s'>{$this->lang->words['topic_start']}</a></li>
<else />
<li class='disabled'><span><if test="isGuestPostTopicTop:|: ! $this->memberData['member_id']">{$this->lang->words['forum_no_start_topic_guest']}<else />{$this->lang->words['forum_no_start_topic']}</if></span></li>
</if>