Looking at the statistic in blockchain, there are 146M addresses with >0 BTC. This will be the upper limit.
As we know, almost all bitcoin users have more than 1 address with >0 BTC; I myself have at least 50 addresses with balance >0 BTC. Some enthusiasts and organizations have thousands of these addresses. My guess is that we are still <1 million users, but not far from that.
This is the most logical way to get a range of how many users there really are.
I guess that's a good number already. My suggestion also, is try to check if the number of users are moving or not. I am pretty sure it's moving, so it's a great sign that we will increase in terms of number and the demand will follow.
Like this way you cannot guess or give any statistical way of giving how many users are their in this world who are using bitcoin and even then what is the use of getting this status until bitcoin is not in the mainstream of business world. We are not even 1% users of the total world population.