Does wicca allow polygamy

Wicca doesn't allow or disallow polygamy. Since Wiccans should abide the law of the country, if the law permits polygamy, then Wiccans would, if they wish so.

In the Wiccan Rede it says, "True in love ever be, Unless they lover is false to thee".

There is no reason for Wicca to disallow (or allow) polygamy - as long as you are honest and loving to each of your partners, it doesn't go against any of the set Wiccan teachings (different traditions may have different views).