There has never been a census done of people who practice Dzogchen, and there probably could not be one done accurately, because for a lot of people it is a secret practice. Therefore, we don't know how many dzogchen practitioners there are on the planet, on the internet, or on this forum, and who their teachers are.
I do know that most of my vajra sisters and brothers would not be interested in talking about dharma on-line, and among the subset who would, most are not interested in being in an atmosphere like Dharmawheel and it's predecessors--a debate-like atmosphere with mostly brainy men participating. I'm not saying that is bad or good.
However, based on my friendship network, Asian and American primarily, I really doubt that there are more Dzogchen practitioners that study under Namkhai Norbu Rinpoche. More than most teachers, probably, but not more than all teachers, or all teachers put together.
Patrul Rinpoche wasnt' found in revealing teachings to everyone like on the open market. Since the dzogchen can be very easy misunderstood and lead to even more selfishness and heedless behaviour.
Namkhay Norbu Rinpoche La like all manifestations is selfless compassionate action, Bodhichitta. Possible now there is internet method, still the warning for misunderstanding remains.
“Sentient beings, self and others, enemies and dear ones—all are made by thoughts. It is like seeing a rope and mistaking it for a snake. When we think that the rope is a snake, we are scared, but once we see that we are looking at a rope, our fear dissipates. We have been deluded by our thoughts. Likewise, mentally fabricating self and others, we generate attachment and aversion.” ~Dilgo Khyentse Rinpoche