how many times Spoils of War can get the discount?

Once per cast, and it's always 2 — no matter how many enemy units died.

Spoils of War checks whether at least one enemy unit died this turn when you play it. If yes, it costs 2 less; extra kills don't stack the discount higher, and there's no limit to how many times you can benefit across the turn as long as the condition holds for each cast (FAQ 2376; Core Rules 356.4).