Tom,
No, you can get that info at a topic level by swapping ‘topic’ for ‘category’ like this:
SELECT
COUNT(topic_id)
FROM
topic_users
WHERE
notification_level = 3
Again, this is a very basic query that just returns a total count of watchers from…in this case…all topics. If you want to specify a particular topic, you could add to the WHERE clause, e.g:
WHERE
notification_level = 3
AND topic_id = 29