You can only pay one mana when resolving Embersmith’s ability.

Unless specified, you are only allowed to pay a specified cost once when an ability resolves to get an effect. Embersmith‘s ability is a an example of this. When you cast an artifact spell Embersmith’s ability will trigger, go on the stack, and you will choose a target creature or player. When this ability resolves you can pay one mana, just once, to have the targeted creature or player receive one point of damage. You cannot choose to deal more than one damage to a single target, or deal damage to multiple targets by paying additional mana.

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