Mana Echoes
Enchantment
Whenever a creature enters, you may add an amount of 
equal to the number of creatures you control that share a creature type with it.
When the ground is saturated with mana, even the lightest footstep can bring it to the surface.
Christopher Moeller
Double Masters (2XM)
• #136Rulings
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}.
(wotc • 2020-08-07)
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}.
(wotc • 2020-08-07)
The amount of mana to add is determined only as the triggered ability of Mana Echoes resolves. The ability isn't a mana ability; players may respond to it.
(wotc • 2020-08-07)
Rulings
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}.
(wotc • 2020-08-07)
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}.
(wotc • 2020-08-07)
The amount of mana to add is determined only as the triggered ability of Mana Echoes resolves. The ability isn't a mana ability; players may respond to it.
(wotc • 2020-08-07)