If a 6th level Sorcerer with Elemental Affinity (lightning) casts Witch Bolt, is only the inital bolt's damage is increased, or the damage of every one of them?
Elemental Affinity:
[...] when you cast a spell that deals damage of the type associated with your draconic ancestry, add your Charisma modifier to that damage.
Witch Bolt:
[...] on each of your turns for the duration, you can use your action to deal 1d12 lightning damage to the target automatically.