We get 4 a week from doing the weekly quests. The first two cache’s you open are supposed to have guaranteed veteran gear (I believe its veteran). The first four you open are guaranteed Restored Coffer Keys.
Although, I felt like I was swimming in coffer keys in season 1, and now it feels like famine. Maybe it’s just me. I know I had skipped using them for a while because my toon didn’t need anything from Delves, but when the season ended I had like 40+ coffer keys. Am I in a fever dream or does it feel like the key drop rate was nerfed with season 2?