It’s been over a year now since Rocket League first launched on PS4 and PC, also it’s only been about 5 months since the game released on Xbox One. The game has made a collector’s edition, introduced cross-play between Xbox and PC players, and has gotten tremendous support since it’s release on Xbox One.
The official Rocket League Twitter account has announced that the game has reached 19 million players:

Congrats everyone! We've officially reached 19 million players for Rocket League! We are humbled by your passion. Thank you!

— Rocket League (@RocketLeague) July 18, 2016

That’s a huge accomplishment for Psyonix, they have done an outstanding job since the game released in 2015.