Hamza, Guardian of Arashin

This spell costs {1} less to cast for each creature you control with a +1/+1 counter on it.
Creature spells you cast cost {1} less to cast for each creature you control with a +1/+1 counter on it.

"I stand with every member of my clan, and they stand with me."

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Hamza's first ability affects only generic mana costs. It can't reduce the total cost to cast the spell below {G}{W}.

Once you announce you're casting a creature spell, no player may take actions until the spell has been paid for. Notably, opponents can't try to change the number of creatures you control with +1/+1 counters.

To determine the total cost of a spell, start with the mana cost or alternative cost you're paying, add any cost increases, then apply any cost reductions (such as that of Hamza, Guardian of Arashin). The total cost is locked in before any costs are paid. The mana value of the spell is determined only by its mana cost, no matter what the total cost to cast the spell was.