Good Times With Weapons - S8-E1
Corrected entry: Kenny only bought one ninja star, and after the fight with Professor Chaos, it's stuck in Butters' eye, so Kenny shouldn't have it anymore. However, when Kenny and the other boys fight the other group of "ninjas", you can see Kenny is still holding a star.
Correction: We are never told how many ninja stars Kenny has bought, only that EVEN he can afford them at $1.99.
Soylent Purple