Legendary Artifact Creature — Golem
* / *
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
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.
The ability that defines Karn's power and toughness works in all zones, not just the battlefield. As long as Karn is under your control and still an artifact, its own mana value will count. In most cases, it'll be at least 5/5.