People tried to estimate the number of bitcoiners around the world, the estimation ranged between 1M and 2M some time ago. If you search you'll find them. Number probably is outdated too.
There is a question, what could be considered as owning a Bitcoin? If someone has bitcoins sitting in an exchange or an y other place that gives no access to private keys, then will he be considered as bitcoin owner? What about blockchain.info, for example?
Just wait for another 5 years maybe 50% of the global population around the world already owns 1 bitcoin. With the price getting higher and higher each day, almost everyone is craving to earn bitcoins.
3.75 billions people each owning 1 Bitcoin, while the supply is limited to 21M? Looks like impossible.