The verse most commonly referred to with the topic of polygamy is Verse 3 of Surah 4 An-Nisa (Women). A translation by Yusuf Ali is shown below:
If ye fear that ye shall not be able to deal justly with the orphans, 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, or (a captive) that your right hands possess, that will be more suitable, to prevent you from doing injustice.-Qur'an, Sura 4 (An-Nisa), Ayah 3