From the Xbox Wire post, XB1 ver of Rocket League has almost 2 million players.
http://news.xbox.com/2016/05/24/cross-network-rocket-league-play-arrives-on-xbox-one-and-steam-pc/
Nearly 2 million Xbox One players have already competed in Rocket League since we launched in February 2016, but with today’s new cross-network update, your potential pool of teammates and opponents will grow even larger!
http://news.xbox.com/2016/05/24/cross-network-rocket-league-play-arrives-on-xbox-one-and-steam-pc/