Nobody, repeat, nobody cares one little bit if you wear the same thing two nights or three or four, as long as you roughly adhere to the dress code on which Norm has elaborated. At the risk of being ridiculously repetitious, nobody cares.
I usually bring one casual shirt and one pair of socks per night plus one spare (in case of cancelled flights, etc., that force an unexpected stay en route home -- or in case a spill, etc., requires an extra change of clothes. A caual shirt worn to dinner on a casual evening should be fine for daytime wear the next day. At the end of the day's activities, there's a shower and a clean shirt.
Although we always adhere to the dress policies (well I cheated two weeks ago on the Millie transatlantic when I wore a tie and sweater instead of a tie and jacket one night), we do not overpack and we mix and match to make things work.
After I wrote the earlier reply, I recalled the outfits that my cousin found for the bridesmaids at her wedding. Each outfit consisted of dark green velvet cocktail or party dresses (knee length) with light green silk wrap skirts (full length) worn over the dress, from the waste down. The wrap skirts were fairly full, the dress underneath gave them some body. The result was a great combination for cruising -- one could wear the whole outfit on a formal evening and the dress alone on an informal or semiformal evening. The wrap skirt also would fold very compactly, being of silk.