You can get the # of subtree notifications to a specific node using this SQL (mysql):
SELECT count( DISTINCT a.user_id ) as count FROM ezsubtree_notification_rule a, ezcontentobject_tree b WHERE b.node_id=NODE_ID AND b.path_string like concat( '%/', a.node_id , '/%' );
If you just need this "once in a while": I have an SQL-statement which does print a list of all users and their "notification nodes" for ez 3.4. I think, this should work for 3.5+, too. But you'll need some access to your DB for this, of course.