Just a word of warning - this unlocked at 332 for me - seems slightly glitched!
The challenges comprise of the following: -
Wolf Creek - 62 Challenges
Paracel Storm - 54 Challenges
Coyote Falls - 58 Challenges
So a total of 522 stars available.
You can get 3 stars in each challenge, so you do not need to 3 star them all - once you get 1 or 2 stars just move on to the next...