Good questions!
The 100 metres / 330 foot maximum depth for Advanced Trimix was the result of some internal meetings and feedback from TDI's senior Instructor-Trainers. It's also a nice round figure
Among the factors considered were the operational limits of OC technology. As you know, a dive to 100 metres on open circuit is about the deepest most would attempt under most conditions without structured in-water and surface support. This support would add security in the event of gas emergencies, such as deep decompression gas loss, among other things.
As for the number of certifications annually world-wide in both OC and CCR, I think we'd need an OK from the BOD before sharing those figures in a public forum. However, here are some observations.
I know several of the folks teaching at this level around the world and the figures are reasonably small compared to something like decompression procedures... just as you would image. But one of my friends diving in warm water, runs Advanced Trimix programs on a bi-weekly basis and certifies several dozen divers at this level each year. When I was teaching full-time, I probably certified up to 20 Advanced Trimix divers and or instructors annually. So it is a popular course.
Outside North America, I'd guess the number of certs at this level on CCR would be close to an order of magnitude greater than on OC. The situation in NA is probably reversed. But I think that ratio will change as time goes on.
Hope this helps
