Time of year and WHERE you are cruising makes a difference. And I'm a guy so that's a HUGE difference.
Generally speaking I wear shorts and a polo shirt during the day - except for times by the pool, on the beach or in the ocean.
RE: Shirts
I used to have a shirt for "daytime" and another one for "nighttime". Well I have gotten pretty good at planning and putting on a clean polo for dinner and then wearing it the next day (or day after) Still only wearing it ONE day, it's just that its fresh in the evening, and not quite as fresh the next time. No one notices or cares that I wore the same shirt twice. Usually we go to dinner, go to the show, go to the adults area at night, so i am NOT pitting out my shirt in the evening. So that helped me cut down the number of shirts by at least 40%. Formal and semi formal nights along with Palo dinner create some bumps in this plan - but you can work around those. I usually bring 2 swim suits but only one tee shirt (cover up).
RE: Slacks
For 7 night cruise I usually bring 3 slacks ... because we usually have a Palo brunch in there so I only wear them 3 times (so there is 1 extra just in case) ... and usually only at meals ... as I will change back into shorts after dinner. If we KNOW that we won't be dining in Palo, then I can get by with only 2 slacks - but that makes me a little nervous.
RE: Shoes
1 Dress, 1 Tennis Shoes (sneakers / trainers), 1 Athletic Sandal. If there's room a pair of flip flops (aka th... wait I'm old). I say if there's room because I've got big stinkin' feet, I mean they are YUGE / Bigly!!
RE: Other (personal items)
This comes down to personal preference and the When and Where you are sailing. Caribbean in July / August - yea, double or triple up every day
NOTE: the Dining Rooms are downright cold year round - so ladies you will need a shawl for a cover up or a light sweater.
RE: When and Where makes a difference
Caribbean & Med - Summer months (including mid May through September) - you will get pitted (I wanted to change after every time day or night that I went outside)!! But you will probably be spending more time by the pool, out on deck, on the beach or in the ocean.
Alaska: Layers - long sleeve shirts, wind breakers, hoodie and a jacket. Earlier and later in the year - the jacket should be heavier and several water resistant items
Northern Europe: More like Alaska, but without the heavier coat (unless you are going to Norway-I assume you might need it, Stockholm / Helsinki / St. Pete not so much)
Pro Tip: 2 weeks before your cruise call
DCL and get the schedule of "theme nights": Casual, Formal, Pirates, etc and work on planning out how to "double up" your tops
Pro Tip: Check the DAILY weather up until you leave to help determine what to take / what to leave home
Last year we were on the cruise from NYC to Quebec but were in NYC for about a week before the cruise. We checked the weather and packed accordingly. "Unfortunately" the weather was clearer (YEA!!!) but about 10 degrees colder (boo hiss) and if we wished we had some warmer coats.
HAVE FUN and good Luck with your Luggage Reduction Process