Stormwing Entity's cost reduction applies just once, no matter how many instant and sorcery spells you've cast this turn.

Stormwing Entity's cost reduction applies even if the instant or sorcery spell you cast was countered or didn't resolve.

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 Stormwing Entity). The mana value of the spell remains unchanged, no matter what the total cost to cast it was.