According to Wikipedia, there are currently 3 ITF Taekwon-do organisations:
- ITF - President Chang Ung
- ITF - President Pablo Trajtenberg
- ITF - President Choi Jung Hwa
Does anyone know how many members each organisation has?
I think this one is going to be way too hard to answer exactly. I did some digging and all I could find was information on the one that I'm assuming is the most popular located in Korea. If you visit ITF Official you can see that their membership climbs to a claimed 14201 members.
This doesn't seem too far fetched as they are also claiming stake in 17 countries. I would assume there are a similar number for the other levels of what you state are apart of this original set up.
There is an ITF Administration that claims 110 organizations. They do not claim a number of members directly on their site that I have found.
Taekwon-do is ever evolving, ever growing. There really could be a count that is created, but doing it separate doesn't make sense to me. In the end all of the forms follow a general baseline and all of the teachings follow a general model. Even if one is the most popular world wide it might not be popular in the region you're in or another reader is in. But knowing these separations are interesting enough!