There is a number of additional Goblins on the battlefield equal to or greater than the number of times you've cast Krenko from the command zone this game minus three.
Steps
Activate Krenko by tapping it, creating a 1/1 Goblin creature token for each Goblin you control.
When the Goblins enter the battlefield, Mana Echoes triggers that many times.
Resolve the first trigger, adding ({C} magic symbol) for each Goblin on the battlefield.
Repeat step 3 for each remaining trigger.
Activate Skirk Prospector twice by sacrificing Krenko and a Goblin token, adding ({R} magic symbol) ({R} magic symbol) .
Choose to return Krenko to the command zone from your graveyard.
Cast Krenko from the command zone by paying ({2} magic symbol) ({R} magic symbol) ({R} magic symbol) plus commander tax, if applicable.
When Krenko enters the battlefield, Mana Echoes triggers, adding ({C} magic symbol) for each Goblin and Warrior on the battlefield.