Anon, it is strictly unique poster count. From 750 posts, it can never be higher than 750, minus baker's posts (not sure if TOR users are differentiated in post count). When the system gets flooded, such as during a stress test, you will be lucky to get a post out within the few minutes before the board fills due to the lurker count and the time it takes to post, and many will be trying to reply in the same thread.
Actual users are not counted, as this is a cloud implementation on many servers, it takes more effort to do an accurate count than the time/effort/cost it is worth.