Most games have the two standard teams, and if you're not on one of those teams actually playing, you're on a spectator team. That means in a 4v4, you've got 4 players on each team, and then you've still got slots left on the spectator team.
The way Rocket League does it is the spectators count toward the match maximum, unless I'm mistaken, meaning that because rocketgaf is full, I can't even spectate. Spectator teams usually ensure you can join a full match as soon as a spot becomes available, rather than sitting around at the main menu like I am, not knowing when and if a slot has opened up.