For us guys, it's pretty easy to do two formal nights. Just bring a second dress shirt and a second necktie to wear with your dark suit. There also will be one or two semiformal nights, for which you will need dress shirts, neckties, and either a sportcoat (to wear with dress slacks) or a light suit.
You might not appreciate this before you go, but you will discover two things aboard the ship.
>> 1. There's something really wonderful about seeing everybody all decked out in their "cruise ship formal" finery and
>> 2. You really won't want to miss dinner in the dining room on the formal nights because there's a direct correlation between the menus and the dress code. On the last formal night, for example, the appetizers generally include caviar (skip it -- tastes like fish eggs!) and the entrees typically include twin lobster tails, roast pheasant, and beef wellington.
Anyway, do have a great cruise!