Muslims are allowed to have up to four proper wives (free women), and as many concubines as they can afford.
And if you fear that you cannot act equitably towards orphans, then marry such women as seem good to you, two and three and four; but if you fear that you will not do justice (between them), then (marry) only one or what your right hands possess; this is more proper, that you may not deviate from the right course. Sura an-Nisa' 4:3
The expression "what your right hands possess" occurs frequently throughout the Qur'an in regard to those women who are "permitted" for Muslim men to have sexual relations with, cf. 4:24, 25, 23:6, 24:31, 33:50, 70:29-30.
Concubines are slave-girls who have either been bought or captured in war. Muhammad himself had four concubines, cf. Mary, the Copt
Morally most reprehensible is the permission given to Muslim men to have intercourse (i.e. rape) female prisoners of war. For details, see the entry on RAPE.