There are “_user” tables.
category_users
, topic_users
, group_users
and tag_users
Each has a notification_level
field where the integer indicates a members preference - 0 Muted, 1 Normal, 2 Tracked, 3 Watched
So a query something like this should work
SELECT COUNT(id)
FROM category_users
WHERE notification_level = 3
GROUP BY category_id
to give you how many are Watching each category.