The cost reduction applies to spells cast by any player.

The cost reduction will be {2} no matter how many times the spell targets the same Spellwild Ouphe.

The spell may have any number of other targets; that won’t affect the cost reduction unless the spell targets other Spellwild Ouphes as well. In that case, the spell costs {2} less to cast for each different Spellwild Ouphe it targets.