The trick is to have clothes that you can mix and match, forget the D.J. and have a suit which you can wear alternative trousers with and suit trousers that go with other things and shirts that will look good with and without ties. Ladies can do the same thing with tops, skirts and slacks. Most of us take more than we ever wear so be strict with yourself to start with but don't miss out on special nights, unless of course that is your bag, as the youngsters say nowadays (I would never dream of saying that). ..... Neil.
On an 11 day cruise there are normally 3 formal nights in the restaurant with RCCI. We did a similar cruise around New Zealand and, due to the luggage weight restrictions, travelling to Australia we did not use the restaurant on the formal nights - we ate in the Windjammer (buffet style). A lot of guests were wearing jacket and tie instead of tuxedo/dinner jackets.
Hope this helps.