If Thieves' Guild Enforcer gains deathtouch only after dealing damage (most likely because another creature was dealt lethal damage at the same time that Thieves' Guild Enforcer dealt damage), the damage it dealt won't be from a source with deathtouch.

Thieves' Guild Enforcer gets +2/+1 just once, no matter how many opponents have eight or more cards in their graveyard and no matter how many cards they have past the eighth.