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.

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}.

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}.