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The cost reduction is based on the total number of +1/+1 counters on Herald of War, not just the ones put on it by its own ability.
If there are additional costs to cast a creature spell, such as a kicker cost, apply those increases before applying cost reductions.
To determine the total cost of a spell, start with the mana cost or alternative cost (such as a flashback cost) you're paying, add any cost increases, then apply any cost reductions. The mana value of the spell is determined only by its mana cost, no matter what the total cost to cast the spell was.
The +1/+1 counter is put on Herald of War before blockers are declared and combat damage is dealt.
A creature spell that's both an Angel and a Human will cost less to cast for each +1/+1 counter on Herald of War.
