The Draconic Bloodline sorcerer's Elemental Affinity feature description (PHB, page 102) says:
...when you cast a spell that deals damage of the type associated with your draconic ancestry, you can add your Charisma modifier to one damage roll of that spell.
The Scorching Ray spell description (PHB, page 273) says:
You create three rays of fire and can hurl them at targets within range. You can hurl them at one target or several.
Make a ranged spell attack for each ray. On a hit, the target takes 2d6 fire damage.
Because I make an individual ranged spell attack for each ray of fire, and I have fire elemental affinity, can I add my Charisma modifier to each of the rays? Would it just impact one of the rays, because it's through one spell cast?