Polygamy can be defined as a marriage which includes more than two partners.
Polygamy in Tamriel
In Tamriel, for both the highborn and lowborn, marriage is chiefly between two people. The Church of Alaxon and its branching-religions such as the Faith of the Eight, do not permit polygamy, and consider it a sin.
Elsweyr
Ancient triban Khajiiti chiefs sometimes took multiple spouses, though the contemporary kingdoms of Elsweyr no longer practice polygamy.
Hammerfell
In Hammerfell, it is not uncommon for a nobleman or a noblewoman to have a spouse for political reasons and a paramour for love. While the Redguards are not very concerned if a child is born out of marriage, children by paramours are nevertheless considered to be bastards, and thus have no right to the parent's surname, while the children by the spouse use the family name and inherit.