You can earn this achievement in custom races by playing solo.
Before starting the race, select any of these two game modes:
-Squad Item
-Squad Speed
When selecting, just play 50 races and you will get the achievement of having won 50 races as a team.
You will also unlock the following achievements:
-Play 1 custom race
-Play 30 custom races
-Win a race for the first time
-Finish in first place a total of 50 times
-Use 500 items in any game mode (item mode only)
If you don't want to play, you can set the game to drive itself and just use the items you get for the use 500 item achievement.
The fastest track in my opinion is: Village: Cobbled Quayside