Is Judaism mono or polytheistic?

Judaism is monotheistic. God is One. The concept of a dualism (as in Zoroastrianism), an independent Satan, multiple gods (polytheism; paganism) or a trinity of three in one, are all unimaginable in Judaism. Also, any belief that an intermediary between humanity and God should be used, whether as necessary or even optional, is considered heretical.He is non-physical, indivisible and incomparable. Jewish tradition teaches that God is beyond human comprehension; and that it is only God's revealed deeds, as He interacts with mankind and the world, that we can begin to grasp.

God is omniscient (He is aware of everything), and infinitely wise.
God created the universe and all existence, including time and space, in a deliberate, purposeful act of benevolent Creation.

God is the mover of everything. No molecule can move without the energy and direction with which God imbues it.

God is eternal; and His ways are also eternal. He is not capricious, forgetful or fickle.
God is just. He rewards good and punishes evil - whether in this world or in the afterlife.

God is ethical and moral; and He expects us to imitate His ways.

The God of Israel is the God of Abraham, Isaac and Jacob. He is the guide of history, who delivered the Israelites from Egyptian slavery.
God is the source of law, who gave the Torah to the Israelites at Mount Sinai.
God is immanent and personal, meaning that he relates to humanity and hears our prayers. This is the basis of the Psalms and our siddur (prayerbook).

Judaism is strictly monotheistic.