Bitcoin could have around 1 to 5 million users in 2020. But that depends on the infrasture of the bitcoin being built.
I think bitcoin has currently around that number and personally I am expecting it to rise due to the rising price which attracts more people who will in turn cause more price rise. It is like a loop and will continue as long as it is let continued. Usually someone just organize some kind of temporal crisis that will make people panic selling their coins and the price drops (like Gox).
Bitcoin is currently on 3 million total of users.
(Check it here: https://www.reddit.com/r/Bitcoin/comments/2vuvxs/estimated_number_of_bitcoin_users/)According to
amnesiac-eightyfourBitcoin owners: probably 2-3 million
Bitcoin users (actively spending, trading, following news): 200k-500k
The (pseudonymous) character of Bitcoin doesn't allow us to make exact figures, so all figures in this thread, including mine, are just guesses or assumptions.
I think in the end it's not so important how many people use Bitcoin. More important is if the infrastructure is getting stronger and if the network activity is growing. I think all indicators show that's true.
I think the active bitcoin users are fewer than 100,000. If there are more than that, and each has more than 2 transactions per day, there will be huge congestion.