I would like to know how many people practice Brazilian Jiu Jitsu around the world.
There's no way of knowing the total number of BJJ practitioners worldwide, because it's a really broad question. The only way I know of for getting the exact amount is to call up each and every BJJ school on the planet and ask them for a head count, including teachers and students. Then you total that up.
There will be error even in that, though, because there are many groups of BJJ practitioners that practice out of their garages or in parks. Lots of colleges and universities now have BJJ clubs. Many high schools even have BJJ clubs. And BJJ groups operate in lots of other unconventional places like dance schools, yoga places, the YMCA, Taekwondo schools, Jewish Community Centers, churches, etc. They're not going to show up in the Yellow Pages or even on the web. Finding them all is an almost impossible challenge.
If you accept that you'll never have 100% accuracy, and that this is inherently going to be an estimate, then you're fine.
But that's still a lot of work.
A better way of estimating this is to figure out how many schools are in a big city, like Los Angeles. Then call up a random sampling of them and ask them for how many people they have. Take the average and multiply that by how many schools there are in total.
That gives you an estimate of how many BJJ practitioners there are in total in Los Angeles. To extrapolate this to the entire country, we need to first figure out the "per capita" rate of BJJ practitioners.
Los Angeles has 13 million people living in it, depending on the area you're looking at. So if you know how many people in total are practicing BJJ in Los Angeles, then you just divide that by 13 million people total to get the per capita rate of BJJ practitioners.
Let's say there are 13,000 BJJ practitioners in total in Los Angeles. Then the per capita rate of BJJ practitioners is 13,000 divided by 13,000,000. This would give a per capita rate of 0.001.
If you know the U.S. population is 324,000,000 people, then you multiply that by 0.001 to get a total of 324,000 people doing BJJ in the U.S. alone. (Again, totally made-up number, don't use for real!)
This is just for the U.S. Now you have to do the same thing for each of the other countries in the world. Each country will have a different per capita rate.
Of course, this is full of statistical and methodological problems which increases the margin of error. In our example above, we just looked at Los Angeles. Los Angeles may or may not be representative of all other cities in the U.S. Other cities may have a different per capita rate of BJJ practitioners.
So you can repeat this analysis by sampling other big and small cities and averaging them to reduce the error.
And instead of analyzing all countries in the world, you could take a similar approach that we took for cities. You would just take a random sampling of different countries and find the average.
The more cities and countries you analyze, the smaller your margin of error becomes, but the more effort you're going to put into it.
I don't have any definitive numbers for you. I just wanted to describe how I would go about it if I were you. It can be done by one person in a fairly short period of time, if you accept that you have a good amount of error in the calculation. If all you want is a ballpark estimate, then you're fine.
Hope that helps.