As many as he can afford to maintain. The Koran allows for polygamy only if the man can keep all wives at the same standard of living and support all the children. EDIT (3/29/07): The Quran in Sura 4:3 says: And if you be apprehensive that you will not be able to do justice to the orphans, you may marry two or three or four women whom you choose. But if you apprehend that you might not be able to do justice to them, then marry only one wife, or marry those who have fallen in your possession. (Maududi, The Meaning of the Qur’an, vol. 1, p. 305) The key clauses are "you will not be able to do justice to the orphans" and "you may marry two or three or four women whom you choose." Maududi (d. 1979) is a respected traditional and conservative commentator on the Quran. So we should let him explain what they mean. He says that the clauses accomplished three things: First, in pre-Islamic Arabia, guardian men married the orphan girls under their care, so the Quran says that they should direct their attention to women other than the orphans. Second, in pre-Islamic days men used to marry a limitless number of women and grab the property of their orphan nephews and nieces to support their wives. Third, in pre-Islamic days, men could marry as many women as they wanted and "treat them cruelly and unjustly" with impunity. So the Quran limits the number to four, and only if the man could keep care of them all: "But if you apprehend that you might not be able to do justice to them." Maududi concludes that probably all three correctives were intended by this verse (vol. 1, pp. 306-07, note 4).

As many as he can support and the law of his country allows.

4 legal wives, and an unlimited number of concubines. The only restrictions are that he must be able to support them.

if u wanna know the real deal then it's 4 wives under strict conditions and in order to do so he haz to be able to support all of them a be equal to each, which is very hard to do so if he married 4 and didnt treat them equally then he didnt abide by the quran.