To calculate this value, you have to take a percentage of people who don't use internet in their daily life, also add here a part of humanity who don't have a computer or electricity at all...I think not many people in the world know about BTC
I will not waste my time to study on this matter, I believe national statistics should work for this one.
Well, I just believe that everyday more people are coming in cryptoworld and I know this is going to happen, more people will enjoy trading and investing with cryptocurrency.