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

That means users of bitcoin compared to the total population is not really that big, we might increase in a yearly basis but we cannot reach the time that everyone will use bitcoin not even in the future.