Titania rewards all who honor the forest by making them a living part of it.

Priest of Titania

{T}: Add {G} for each Elf on the battlefield.

Modern Horizons 3

The List

Commander Anthology

{T}: Add {G} to your mana pool for each Elf on the battlefield.

Commander 2014

Priest of Titania's ability counts all Elves on the battlefield. This includes Priest of Titania itself as well as Elves controlled by other players.

Priest of Titania's ability is a mana ability. It doesn't use the stack and players can't respond to it. Notably, this means other players can't try to remove Elves from the battlefield after you activate this ability but before it resolves.