Karn, Legacy Reforged [March of the Machine: The Aftermath]

Karn, Legacy Reforged's power and toughness are each equal to the greatest mana value among artifacts you control.

At the beginning of your upkeep, add {C} for each artifact you control. This mana can't be spent to cast nonartifact spells. Until end of turn, you don't lose this mana as steps and phases end.