No one can answer this correctly however i think atleast 1% or almost near to that of persons who uses internet banking will be using bitcoin by that 2022. But this number may even go higher if few good development as well as few more open minded politician come into power.
The current active user number is less than 1 million. So if the user number rises 50% a year, it will be around 8 million users then.
We cannot ensure that the figure is true because in the first place bitcoin is anonymous and any person can create different bitcoin address. So if you talking of 1 million, I think that is too big population for a number of bitcoin users.
I guess that is still not enough if bitcoin has only 1 million users, we are a lot of people in this world and I guess if all people on Earth are going to use bitcoins, that is going to affect the demand of bitcoins because of its limitation. For sure that is going to help bitcoins to keep on pumping because many are going to lookafter it.