I couldn't give you specific numbers, but there's only a few worldwide organizations that I can think of, none of which were aware of each other before the reveal.
- The Association/Atlas/the Sea of Estray is the largest group of organized magic users. Mostly European due to their roots there, with a weaker presence in North America and the Middle East and essentially none in South America/Africa/Australia/Asia.
- The Devil Hunter clans are scattered around in small handfuls here and there depending on where the families have moved over the years. Few of the individual clans are in contact with each other, but most know in general terms that there are other clans out there somewhere.
- The Phantasms are loosely scattered around the world but tend to be more nomadic than the Devil Hunters because of the nature of their work. They work solo or in small groups of three or four. They may as well not exist outside of their specific duties since they don't bother with the affairs of other groups.
- The Assassins (pending approval from the rest of the project, since they're kind of out there as magical girls go) have the same deal as the Phantasms but are more likely to fight solo with a small support network of friends backing them up rather than have a small team of fighters.