Cavern-Hoard Dragon

Creature — Dragon

This spell costs less to cast, where X is the greatest number of artifacts an opponent controls.

Flying, trample, haste

Whenever this creature deals combat damage to a player, you create a Treasure token for each artifact that player controls.

Once a player has announced that they are casting Cavern-Hoard Dragon, no player may take actions to try and change the greatest number of artifacts an opponent controls before Cavern-Hoard Dragon's cost is locked in.
Cavern-Hoard Dragon's first ability counts only the artifacts controlled by the opponent who controls the greatest number of artifacts among your opponents. If you have an opponent who controls two artifacts and another opponent who controls three artifacts, Cavern-Hoard Dragon's mana cost would be reduced by {3}.
If the greatest number of artifacts an opponent controls is seven or more, Cavern-Hoard Dragon costs {R}{R} to cast.
Cavern-Hoard Dragon's last ability cares about the number of artifacts that player controls as the ability resolves.