How many Geared 36er Owners/Riders are there in the world?

I would say: “not a lot” :grin:
You can approximate the answers to your questions based on this page:

Of course, not all hubs are listed, neither all the lines mention the uni the hub had been built in. However, it’s probably the best approximation you can get right now. I count around 70 riders possessing a G36er that are listed in that page. I would say they are maybe three times that number overall, so around 200 riders.
Regarding the number of people who don’t own a geared 36er but can ride one proficiently, I would say it is very minimal. It requires a lot of training to be proficient on such a wheel, so it’s really hard to be proficient without owning one.