@niemyjski I was looking at the test EnvironmentInfo_CanRunInParallel()
it runs a Task.StartNew. As far as I remember from a course I once followed the threadpool will be exausted pretty quickly. So the number of threads will be not 10000 but the max avail number of threads in the threadpool.
If you continue to push the CLR for new threads the number of available threads will steadily increase over time.