"Marry women of your choice, two, or three, or four; but if ye fear that ye shall not be able to deal justly (with them), then only one." [Al-Qur'ân 4:3]
Before the Qur'ân was revealed, there was no upper limit for number of wives and many men had scores of wives, some even hundreds. Islam put an upper limit of four wives. Islam gives a man permission to marry two, three or four women, only on the condition that he deals justly with them.
In the same chapter i.e. Surah Nisa verse 129 says: "Ye are never able to be fair and just as between women...." [Al-Qur'ân 4:129]
At that time and even now during wars, there are more men killed as compared to women. More men die due to accidents and diseases than women. The average life span of females is more than that of males, and at any given time one finds more widows in the world than widowers. Marrying a second wife or even a third MUST BE for a reason either Charity (save someone's life such as save a widow woman with or without child or divorced woman with or without child) Because in Prophet's time even now in our Arabic socity it is sometimes Shame for the parents to have their divorced or widow daughter back to their home and they don't support or appreciate it. The woman will not have much support. The other reason man can marry another woman is if his wife cannot bring a child for him OR she has been diagnosed with some disability/illness which makes her unable to perform sex, parenting, or any of her chores as a wife. Also the first wife HAVE TO KNOW AND APROVE of the second marriage if the man wishes to marry another. If the man decide to marry another woman or women he has to be just giving every woman a home of her own, give the same allowance to each one and the same physical attention to each one .. no one more than the other otherwise it's a sin. "if ye fear that ye shall not be able to deal justly (with them), then only one."
Prophet Mohamed for example. His first marriage took place when he was 25 years of age and he married Khadija who was twice widowed, and was 40 years old. Until his first wife, Khadija was alive, he never took a second wife. Khadija died when the Prophet (pbuh) was 50 years old and only after this, did he marry the others. The others were either widows or slaves (to free the slave and give her rights).