2% of the total world population = 148000000
I think somewhere around 0-2%.
60% of the world does not have internet access as of 2016, let alone cryptocurrencies.
So, I guess the total number of Bitcoin users will be somewhere between 0 and 148000000

I guess they should even be more than 2% because of the incessant popularity and acceptance of bitcoin adoption that too happening very rapidly, the number of people using bitcoin should be reaching like 5% of the total world population which is over 200 million people by 2020.