Allaah has allowed men to have more than one wife, as He says (interpretation of the meaning): 

“And if you fear that you shall not be able to deal justly with the orphan girls then marry (other) women of your choice, two or three, or four; but if you fear that you shall not be able to deal justly (with them), then only one or (slaves) that your right hands possess. That is nearer to prevent you from doing injustice”

[al-Nisa’ 4:3]

This clearly shows that plural marriage is permissible, and that according to Islamic sharee’ah a man may marry one, two, three or four wives, but it is not permissible for him to marry more than four. This is the view of the mufassireen and fuqaha’; the Muslims are unanimously agreed on this point and there is no difference of opinion. 

It should be noted that there are conditions attached to plural marriage: 

1 – Justice and fairness. 

Because Allaah says (interpretation of the meaning): “but if you fear that you shall not be able to deal justly (with them), then only one” [al-Nisa’ 4:3]. This verse shows that justice is a condition of plural marriage being permissible. If a man fears that he will not be able to treat his wives fairly if he marries more than one, then it is forbidden for him to marry more than one. What is meant by justice here is treating all wives equally with regard to spending, clothing, staying overnight and other material matters which are within his ability to control. 

With regard to justice and equal treatment in the area of love, he is not held accountable for that, and it is not required of him because it is not possible to control it. This is what is meant by the words of Allaah (interpretation of the meaning): 

“You will never be able to do perfect justice between wives even if it is your ardent desire”

[al-Nisa’ 4:129]

i.e., with regard to love of the heart. 

2 – Being able to afford spending on plural wives. 

The evidence for this is the verse in which Allaah says (interpretation of the meaning): 

“And let those who find not the financial means for marriage keep themselves chaste, until Allaah enriches them of His Bounty”

[al-Noor 24:33] 

In these verses Allaah commands the one who is able to get married but cannot afford it to remain chaste. One of the reasons for not be able to afford to get married is not having enough to pay the mahr (dowry) or not being able to spend on one’s wife. Al-Mufassal fi Ahkaam al-Mar’ah, vol. 6, p. 286. 

Some of the scholars are of the view that plural marriage is better than restricting oneself to one wife. Shaykh Ibn Baaz (may Allaah have mercy on him) was asked: Is the basic principle concerning marriage plural marriage or marriage to one wife? He replied: The basic principle in sharee’ah is plural marriage for those who can afford it, if there is no fear of injustice, because of the many interests that are served by that, such as keeping himself and those whom he marries chaste, kind treatment of them, producing lots of children and increasing the numbers of the ummah and the numbers of those who worship Allaah alone. This is indicated by the words of Allaah (interpretation of the meaning): 

“And if you fear that you shall not be able to deal justly with the orphan girls then marry (other) women of your choice, two or three, or four; but if you fear that you shall not be able to deal justly (with them), then only one or (slaves) that your right hands possess. That is nearer to prevent you from doing injustice”

[al-Nisa’ 4:3]

And because the Prophet (peace and blessings of Allaah be upon him) married more than one wife, and Allaah says (interpretation of the meaning):

 “Indeed in the Messenger of Allaah (Muhammad) you have a good example to follow”

[al-Ahzaab 33:21] 

One of the companions of the Prophet (peace and blessings of Allaah be upon him) said, “As for me, I will not eat meat,” and another said, “As for me I will pray and never sleep”; another said, “As for me I will fast and never break my fast,” and another said: “As for me, I will never marry women.” When news of that reached him, the Prophet (peace and blessings of Allaah be upon him) addressed the people. He praised Allaah then he said: “You are the ones who said such and such. As for me, by Allaah, I fear Allaah more than you do, but I fast and I break my fast, I pray and I sleep, and I marry women. Whoever turns away from my Sunnah has nothing to do with me.” These great words of the Prophet (peace and blessings of Allaah be upon him) include both marriage to one woman and plural marriage. 

Majallat al-Balaagh, issue no. 1015; Fataawa ‘Ulama al-Balad al-Haraam, p. 386. 

See also the answer to question no. 14022.