Wording of a Demon Pact explained

Hoping someone can help me make sense of this wording...

"Whenever your combat spells regenerate fire runes, the damage of your next spell hit is increased by 1 per rune regenerated"

My question is... Is this saying every rune regenerated creates the +1 damage. E.g. fire surge uses 18 runes so can get up to +18 damage. or is it specifically every fire rune regenerated (+10 damage)

I get that more information will come to light but this is just a vague description that has an impact on the path I will be taking.

Thank you!