I'm wondering how many GTD users are in the world.
Do you have any idea on how I could estimate this number ?
What criteria do you want to use for a "GTD user"? It's probably roughly something like (book sales)/(3 or perhaps even 2). There are a lot of people who have heard of GTD on the web, but have not read the book, but there are other people who have the book but haven't read it.
Um, defining "GTD users" as "people who know what GTD stands for" seems like quite a stretch to me.... As does extrapolating a small sample of a technology-oriented workplace to the population as a whole.