This achievement is for participating in 50 events in the calendar in Career mode.
Below are the events that count towards this:
-Extreme Condition
-Training
-Maintenance
-Historic
-Manufacturer Tryout
-Manufacturer Challenge
-Rally
Essentially, all of the events you can select on the calendar except for Rest events count for this.
What I did was start in WRC 2 and only participated in events that were mandatory (2 Training and 1 Extreme Conditions event at the start of your WRC 2 career and 1 Maintenance at the start of your WRC 2 Pro career), all the rallies except for 4-5, one Historic Event and one Rare event for the achievements related to them, and a maintenance after every 2 rallies to keep my car's condition up. There may have ben a mandatory Manufacturer Tryout in there as well, but I don't exactly remember. All the other events were rest events.
Doing this, I earned this achievement with minimal effort right towards the end of my first WRC season. As soon as you start your 50th event, the achievement will unlock.