“Persistent. When Venna attack a region that has a Hag, Acolyte, or Cauldron, deal +1 damage.”
Does having multiple Venna elites in a region multiply the damage dealt by Venna? (no)
This feat works the same whether the region being attacked has a single Venna elite or multiple Venna elites. Having multiple elites in the region does not multiply the damage dealt, it is always just +1 damage if there is at least one Venna elite in the region.