Marry as many women as you'd like -- two, three, or four.

Marry of the women, who seem good to you, two or three or four. Quran 4:3

You may exchange one wife for another.

And if ye wish to exchange one wife for another.... 4:20

But don't marry any married women -- unless they are slaves, then it's OK.

All married women (are forbidden unto you) save those (captives) whom your right hands possess. 4:24

But you won't be able to treat them equally, no matter how hard you try.

Ye will not be able to deal equally between (your) wives, however much ye wish (to do so). 4:129