Julia Layton in How Stuff Works:
Plural marriage is as old as the Bible. Abraham and Jacob each had more than one wife. King David had six. King Solomon had 700 (not to mention 300 concubines). Solomon lost God's favor when he married women who did not give up idolatry, David when he sent a woman's husband to the front lines so he could marry her. Whether ancient or modern, polygamous or monogamous, marriage has rules. There may be ages and genders to consider. In early America, there were races to consider. Often, those considerations draw on religious beliefs: The Quran allows a man to take up to four wives. In Fundamentalist Mormonism, there is no set limit to the number of wives in one marriage. Joseph Smith, the Mormon Prophet who first delivered God's directive that Mormons practice plural marriage, ultimately took dozens of wives.