According to Jeremy Dunham, Vice President of Psyonix, 8 million copies of Rocket League have been sold.
Dunham also stated that 22 million players are currently playing the game as well. This includes both individual and split-screen players. It is worth mentioning that it’s very hard to know just how many copies have been sold since Steam had a free weekend back in April.
If you include the numbers of the free weekend, there are 4.4 million owners from Steam. Many also received the game for free on Playstation 4 via Playstation Plus.