I don't think there is an accurate way of counting as to how many people use Bitcoin other than a compulsory survey. You take a global survey and ask each and every person in the world if they use or know bitcoin.
I think, the survey can not count the number of users bitcoin. Because we know the surveys are boring thing and some people if meets survey they would tend to avoid it. And I think the survey is not an appropriate thing to know about it.
I think the number of users bitcoin could never be calculated precisely, if bitcoin itself still can not be accepted by the government. because help government with data collection, the number of users bitcoin can be done.
Thank you
Bitcoin was made for people to use it anonymously and surveys will be highly dreaded by many people but I might estimate bitcoin users to be approximately 5 million out of the 7.5 Billion (world population) with china having the most users.