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}.
07-08-2020
wotc
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}.
07-08-2020
wotc
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.
Random a card of Magic: The Gathering (MTG), but you can also roll the D20 dice, random a Planeswalker, have fun with the random rules and more.