If you open your challenger cache chest it gives you a mythic+ key one level less than you completed that week.

But lets say you would rather start with all +2 keys. Which I do because there are very few dungeons I can complete on high mythic+ and darn RNG seldom gives those keys to me. I would miss out on a lot of loot unless I started with +2 keys only.

Before opening your chest, run a mythic. It gives you a +2 and since the keys are unique you will not get another when you open the chest.

Thought I would post this in case anyone else found it useful.