There is a mistake, remove the ; from the first line.
You could add this code to your showthread.php
PHP Code:
$postcount= $DB_site->query("SELECT count( * )
FROM vb3_post As p, vb3_thread As t
WHERE p.threadid = t.threadid
AND t.forumid In(16,17,18)
AND p.userid = $post[userid]
Then add $postcount to your postbit_legacy template whereever you want the number to appear.
I this example we are counting posts in forum 16, 17, 18.
Note that this would run a query for each post in the thread everytime anyone viewed any thread.