Well although I don't really get the point of trying to find the total number of bitcoins holders/users , I have something to say.
YOU CAN'T.
You simply can't. Why ? Well, for a lot of reasons.
1- Nearly each and every person has more than one address and since there is no limit on addresses you can have it's UNLIMITED. and I'm just talking about normal individuals.
2- BOTS. Casinos and many other companies have a lot of codes/bots which generate addresses for one-time purchases/deposits. So that's just thousands and thousands of addresses generated. How are you going to determine which is whose ? Exactly...
3- I really can't think of number 3
I think this pretty much explains it.