A qualification of Mohamed as a prophet is that he can do what his people are not allowed to do, by a decree straight from Allah.
According to Muhammad, Allah had commanded that a man could marry up to four women at any one time, hence in Koran we read:
Marry women of your choice, two, or three, or four (Sora 4:3).
Four women were not enough for Muhammad, so he needed special permission from Allah to marry any number of women he wished. The revelation came as follows:
O Prophet! We have made lawful to thee thy wives to whom thou has paid their dowers; and those whom thy right hand possesses out of the captives of war whom Allah has assigned to thee; and daughters of thy paternal uncles and aunts, and daughters of thy maternal uncles and aunts, who migrated with thee; and any believing woman who gives herself to the Prophet if the Prophet wishes to wed her - this only for thee, and not for the believers [at large]; we know what we have appointed for them as to their wives and the captives whom their right hands possess - in order that there should be no difficulty for thee. And Allah is Oft Forgiving, Most Merciful (Sora 33:50).
More about Mohamed's women can be found on the following page:
Questions regarding the Quran and Islam generally