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Whenever a creature enters the battlefield, you may add an amount of {C} equal to the number of creatures you control that share a creature type with it.
Mana Echoes counts the number of creatures that share a type, not how many types they share. For example, if a Human Warrior enters while you control two Humans and two other Human Warriors, you'll add five {C} to your mana pool.
The amount of mana to add to your mana pool is determined only as the triggered ability of Mana Echoes resolves. The ability isn't a mana ability; players may respond to it.
The creature entering the battlefield shares a creature type with itself if it has any creature types, so Mana Echoes usually adds at least {C}.
