World of Warcraft: Current Year

The "+50 achievement" is "Putting the Finite in Infinite", Wowhead says you need to reach 999 stacks of Unbound Limits on your Artifact AND complete a timed +50 or above M key, but just the key is enough to get it.

I'm currently at 543 stacks / versatility after today's Mythic raids, I should be at 800+ in 10 days.