Ahura Mazda is one god recognized.Ahriman is another.These two are opposed,but in this opposition there is balance.
Zoroastrians believe that there is one universal and transcendental God, Ahura Mazda. He is said to be the one uncreated Creator to whom all worship is ultimately directed ,Ahura Mazda is the beginning and the end, the creator of everything that can and cannot be seen, the Eternal, the Pure and the only Truth. In the gathas, the most sacred texts of Zoroastrianism thought to have been composed by Zoroaster himself, the prophet acknowledged devotion to no other divinity besides Ahura Mazda. So their for in Zoroastrianism like Christians theirs only one god they only believe in..
They believe that there is only one God.