You've got to play 90 games in Division Rivals to earn this.
There are 3 Milestone sections.
Milestone 1 = 20 games played
Milestone 2 = 50 games played
Milestone 3 = 90 games played
To see how many you've played, go into the menu where you'd start searching for a Rivals match, and underneath the "Play Rivals" tab it'll tell you your milestone progress