I think it's 2 millions of members that we have right now, or more. That is only to my rough estimation. And in my observation we have member here that has multiple account in bitcoin talk.org meaning the 2 millions can possible to multiply in to 2 or more.
bitcointalk is not a very good measure to the bitcoin world. also making wild assumptions about multiple accounts is no help and the reason why people have no good number and take wild guesses. try a more reasonable approach. like asking how many wallets are there with bitcoin on them.