The standard traditional answer is 'One man is cleaved to one women.', as in Adam and Eve. If you noticed, this does answer the question directly. Yet, this answer alludes that polygamy is sin. If a man died leaving a wife with no children, one of his brothers was duty-bound to marry the widow. In the West, polygamy is labeled a sin.
Western polygamy creates a 'stumbling block' which really is sin.
Polygamy, though illegal in some countries, is not sin. Yet in current times, polygamy can result in a 'stumbling block'. If we create stumbling blocks for new Christians, we have sinned indeed. In Western countries, promotion of polygamy is a stumbling block. In Arab countries demonization of polygamy is a stumbling block.