From the Super Mario Wiki
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Key Coins are a special type of coin that first appear in Super Mario 3D World. Five Key Coins have to be collected to unlock locked Warp Boxes, similar to how Mario has to collect five Star Chips in the Super Mario Galaxy games to make Launch Stars appear. A counter on top of the screen shows how many Key Coins have been collected and how many are left. Collecting a Key Coin gives the player 1000 points.
Key Coin Locations