Laura Betzig illustrates how widespread polygyny was in history.
The Jews were polygynous. The Old Testament recounts
Abraham had a son by his wife, Sarah; he had another son by Hagar, who was Sarah’s maid; and he had 6 sons by Keturah, after Sarah died. But some of his grandsons outdid him. Jacob had 12 sons by 4 women—6 by his wife, Leah; another 2 by his wife, Rachel; and another 4 by Bilhah and Zilpah, his wife’s maids.
David lived in an ivory palace, surrounded by ladies of honor and virgin companions; the Bible names one of his daughters, and 19 of his sons. But his son, Solomon, was much more ambitious than he was. “He had seven hundred wives, princesses, and three hundred concubines; and his wives turned away his heart” (1 Kings 11:3).
The Hindus were polygynous. The Mahbharata tells us
pretty girls dance “by the thousands” when Arjuna marries the flawless princess, Draupadi. And the lord Krishna, whose 8 wives give him 80 sons, captures thousands of women from the evil Narakasura. After which, his first wife is flattered: “You live long with tremendous beauty and welfare. You are considered and worshipped as eldest of all 16,000 wives of Krishna.”
But in Europe, things were somewhat different. Charlemagne had several concubines and 4 wives. Wikipedia says Charlemagne had eighteen children over the course of his life with eight of his ten known wives or concubines. But the 4 legitimate wives were successive not concurrent. This was serial monogamy with some concubinage. It was not a harem.