Andrei4ik93, добрался до компа
$sql = mysql_query("SELECT user.*, COUNT(forum_post.id) AS cnt FROM user JOIN forum_post ON (user.id = forum_post.id_user) GROUP BY user.id order by `cnt` desc limit 20");
while($row = mysql_fetch_assoc($sql)) {
echo '<div>Пользователь: <a href="/user/id=' . $row['id'] . '">' . $row['nick'] . '</a> Постов: '. $row['cnt'].'</div>';
}