I'd suggest maybe 20,000 members (probably fewer), most of which have never posted or have only posted a few times. Almost all of the rest are bots and sockpuppets. The tiny remaining few are the people like you and me who read and post.
Of course, there are also lurkers who've never made an account, but who read the boards.
There are currently 37,928 registered members, not all are active as you say.