I'll venture a guess that I can say is wrong, for certain.
Based on the US population, and extrapolated for the world population...
80,000,000 people.
Now, that would be if there were 4 million in the US, and every single nation had the same ratio as the US.
I'd say between 6 and 12 million is much closer to the real number, but there's no easy way of ever knowing, barring widespread internet and a crazy, crazy billionaire who wants to blow his money on finding things like that out.