“If Venna is damaged by an attack in a region that has a Hag, deal 1 damage to the attacker in that region.”
Do the Venna get to deal multiple damage back to the attacker if the region has multiple Hags? (no)
The Venna deal 1 damage back if the region has at least one Hag. Even if the region has multiple Hags, only 1 damage is dealt by the Venna to the attacker.
Does the attacker need to be in the same region as the Hag in order to be damaged by this ability? (yes)
The attacking faction must have troops in the same region as the Hag in order for the Venna to deal this damage back. This is what is meant by “the attacker in that region.” Since “the attacker” is a faction and not an individual troop, it is enough that the faction that attacked you has troops in the same region as the Hag, even if the attack technically originated from another region. The Venna choose which troop in the Hag’s region from the attacking faction to deal the damage to.