Psyonix, Rocket League developer, has shared that the game has passed 14 million players mark across all platforms. The developer has informed about the success of the game on Tweeter. It's no wonder that Rocket Leagues is so popular. The developers have traveled a long way, starting with the release of Supersonic Acrobatic Rocket-Powered Battle-Cars (for PS3, in 2008), before they succeeded. Here is what Wiki has to say about the game's success:
"Rocket League has been critically praised, earning a number of industry awards, and saw more than four million sales and at least 12 million unique players by early 2016. Rocket League has also been adopted as an eSport, with professional players participating through the Electronic Sports League and Major League Gaming. In March 2016, Psyonix announced the Rocket League Championship Series (RLCS), which is an officially sponsored tournament league. The first season of RLCS will have a prize pool of $75,000 USD."