on what conditions is it allowed for a muslim to marry more than one wife?
Praise be to Allaah.
Marrying more than one wife is a matter which is recommended, subject to certain conditions: that the man be financially and physically able for it, and that he be able to treat his wives justly.
Plural marriage brings a lot of benefits, including protecting the chastity of the women whom he marries, bringing people closer together, and producing a lot of children. This is what the Prophet (peace and blessings of Allaah be upon him) referred to when he said: “Marry the one who is loving and fertile”. And it serves a lot of other purposes. But for a man to marry more than one wife by way of boasting and taking that as a challenge to prove himself, this is a form of extravagance and extravagance is forbidden. Allaah says (interpretation of the meaning):
“and waste not by extravagance. Verily, He likes not al-musrifoon (those who waste by extravagance)”[al-An’aam 6:141]