Ok I gave the request above as I saw you put LIMIT 10 and didn't put a coherent where clause.
What do you want exactly. A number of posts ?
PHP Code:
$getnewposts = $DB_site->query_first("SELECT count(*) as count FROM " . TABLE_PREFIX . "post as p
LEFT JOIN " . TABLE_PREFIX . "thread as t ON (t.threadid = p.threadid)
WHERE t.forumid = 13 ORDER BY p.dateline DESC LIMIT 10");
It's better but, this request will give you the last 10 posts in forumid 13. You must give in the where clause a dateline condition. LIMIT 10 is useless unless you want to display at least 10 last posts and no more
PHP Code:
WHERE t.forumid = 13 AND p.dateline > $bbuserinfo[lastvisit]