A Mormon is only allowed to have one wife, according to the Book of Mormon. Some Mormons marry and have more than one wife, but this is not accepted by the Mormon Church, which does not accept polygamy or plural marriage.
According to the Mormon Church, which is officially known as the Church of Jesus Christ of Latter-Day Saints, polygamy or plural marriage was practiced a long time ago for specific reasons, as commanded by God or the Lord. The church's founder, Prophet Joseph Smith is one who followed this command and was reputed to have had 40 wives. Later, God changed the command, as instructed to Prophet Jacob and mentioned in the book of Mormon as Jacob 2:27-30, "for there shall not any man among you have save it be one wife: and concubines he shall have none...for if I will, saith the Lord of Hosts, raise up seed unto me, I will command my people; otherwise they shall hearken unto these things."