When did polytheism reach its apex?

Pretty early on; perhaps in Abraham's lifetime, some 3800 years ago.

At the time of Abraham the Hebrew, the area where he lived was full of pagan cults; they were polytheistic, worshiping multiple deities. Abraham was the first to advance the idea of ethical monotheism: the worship of One God, and the appropriate ethical code of conduct. See also:

How did polytheism start?

What did Abraham teach?