Mana Echoes's ability triggers whenever a creature, controlled by any player, enters the battlefield. And when that ability resolves, it's "you" (that is, whoever controlled Mana Echoes when its ability triggered [C.R. 109.5]), not necessarily that creature's controller, that may add mana (compare Mana Echoes with Pandemonium); however, the amount of mana added this way doesn't depend on the number of creatures controlled by any player other than "you".
If I'm playing Edgar and I have 5 vampires out and then I cast a vampire and get a vampire token from Edgar, would that trigger Mana Echos twice giving me 13 colorless mana?
If I'm playing Edgar and I have 5 vampires out and then I cast a vampire and get a vampire token from Edgar, would that trigger Mana Echos twice giving me 13 colorless mana?
Mana Echoes's ability will trigger when the Vampire token created with Edgar Markov enters the battlefield (this is in addition to the ability triggering when the Vampire spell resolves and enters the battlefield, assuming it enters as a Vampire creature).
Here is how this scenario can play out.
You control Edgar and Mana Echoes, and five other Vampire creatures with no abilities, on the battlefield, or you control Mana Echoes and five other Vampire creatures with no abilities on the battlefield and own Edgar in the command zone.
You cast Pulse Tracker. Edgar's first ability triggers and goes on the stack.
All players pass, then that ability resolves. You create a Vampire creature token, making Mana Echoes's ability trigger.
All players pass, then that ability resolves. You choose to add C for each Vampire creature you control (so, e.g., CCCCCCC if you control Edgar, the Vampire token, and five other Vampire creatures on the battlefield).
All players pass, then Pulse Tracker resolves and enters the battlefield (C.R. 117.4, 608.3), making Mana Echoes's ability trigger.
All players pass, then that ability resolves. You choose to add C for each Vampire and/or Rogue creature you control (so, e.g., CCCCCCCC if you control Edgar, the Vampire token, Pulse Tracker, and five other Vampire and/or Rogue creatures on the battlefield).
EDIT (Mar. 29): Change card in example scenario.
EDIT (Oct. 31): Edited, including because one rule was renumbered with Core Set 2020.
I'm thinking about adding it to my Edgar edh deck.
Here is how this scenario can play out.
EDIT (Mar. 29): Change card in example scenario.
EDIT (Oct. 31): Edited, including because one rule was renumbered with Core Set 2020.