Dota has reached 180,000 Immortal Players.
According to https://www.opendota.com/distributions, Dota has more than 180.000 Immortal Players. That is 3.15% of the total active playerbase.
I wonder if there is any other game where so many people have the highest achievable rank. The equivalent to flat Immortal in League for example would be Emerald 2. Emerald is the 5th highest rank in the game.