How to displaying advert after first post, 15th post and 29th post?

[kh99]

There doesn't seem to be, but there is a global $posts that is the result of the post query, so you could do something like:

global $db, $posts;
$rows = $db->num_rows($posts);

in a plugin, of course. There are other checks before posts are displayed so I guess there might be situations where there are more rows than posts, but I suppose if it works sometimes, that's better than not checking at all.