Here is the full list of all 50 Monster Energy Supercross - there are The Official Videogame 2 achievements.
Complete a Main Event in a Custom Track created by another player
Validate a total of 5 Custom Tracks
Win a Main Event with Advanced Physics in any game mode
Earn a total of 1,000,000 SX Credits
Get a Holeshot in any game mode
Get a total of 20 Holeshots in any game mode
Unlock all the Compound tracks
Win a Main Event with the Race Type option set to "The Real Thing"
Purchase a total of 10 components on the Rider Customization page
Win a 450 Championship in Career mode
Win a 250 West Championship in Career mode
Win a 250 East Championship in Career mode
Reach the podium in 100 Main Events in any mode
Reach Prestige level 50
Complete at least one lap using the Helmet Camera in any game mode
Set the pole position 30 times in Qualifying session in any game mode
Reach a Sponsor Objective in Career mode
Keep the same Sponsor for an entire 450 Season in Career mode
Win a Challenge against a Rival rider in Career mode
Win a Challenge against a Neutral rider in Career mode
Participate in a total of 5 Fame Activities in Career mode
Cover 100 miles (160 km) in Free Roaming in the Compound
Win a Main Event in Anaheim 1 in any game mode
Win a Main Event in Anaheim 2 in any game mode
Win a Main Event in Houston in any game mode
Win a Main Event in Glendale in any game mode
Win a Main Event in Oakland in any game mode
Win a Main Event in San Diego in any game mode
Win a Main Event in Arlington in any game mode
Win a Main Event in Tampa in any game mode
Win a Main Event in Atlanta in any game mode
Win a Main Event in Daytona in any game mode
Win a Main Event in St. Louis in any game mode
Win a Main Event in Indianapolis in any game mode
Win a Main Event in Seattle in any game mode
Win a Main Event in Minneapolis in any game mode
Win a Main Event in Foxborough in any game mode
Win a Main Event in Salt Lake City in any game mode
Win a Main Event in Las Vegas in any game mode
Participate in a total of 5 Sponsor Days without changing Sponsor in Career mode
Get at least 10 Stars for every Rider Skill
Get the maximum number of Stars for any one of the Rider Skills
Create a custom start module in the Track Editor
Complete a Championship with any class in Official Championship mode
Get a Holeshot and victory in the same Main Event in any mode
Complete a total of at least 100 scrubs
Beat the time of the Ghost of another player in Time Attack mode
Complete at least two SX Challenges
Reach Prestige level 100
Win all the Main Events of the Triple Crown events of a 450 Championship in Career mode