Luggage
The ancient steamer trunk was today's suitcase, but larger & more cumbersome for your man-servant to handle. It carried the bulk of what you were taking.
Today's suitcase is usually smaller with efficiency-packing compartments and mobility features: rolling wheels, extendable handles, and top/bottom 'grab handles.'
Most are prettier than mine (at right) which has been dragged down innumerable dirt roads, over rough cobble-stone streets, tossed on train & bus luggage racks all over the world for 20+ years, AND modified & repaired several times.
The day-pack is your suitcase's 'little brother or sister ', a useful airline carry-on AND for carrying your Shore Excursions essentials.
Everything should be in those TWO
I will focus only on 1 suitcase & 1 daypack.
Anecdote: Camino de
Santiago: for my 600-mile trek across north-central Spain, I purposely carried only a medium-sized day pack of essentials, relying on albergues (refugios, nunneries & monasteries, etc, restaurants & cafes along the way.
Resist the belief that others gauge your travel skills or character & status based on the newness, cost, or flashy look of your luggage. I never dwelt on others' luggage. If they dwelt on mine, it was none of my business & I didn’t care. I doubt anyone, but thieves do care; fancier bags attract thieves.
Please Note: Unlike big bus tours which require you to often drag ALL your luggage a block or so, & then upstairs each evening & morning, cruiser guests merely have to get their luggage to the ship & ship’s crew gets luggage to your cabin.
A. So, Luggage has 4 parameters: 1) type, 2) size big, 3) number, & 4) durability.
Some haul a couple of suitcases, a handbag/briefcase with other bags dangling off their arms. I have often seen those extra dangling bags left behind on a plane’s overhead, bus seat or hostel dorm floor.
Quote: “Take half the stuff and twice the money”, Susan Heller.
1. Type of luggage?: my ideal combo: a suitcase & a daypack.
2. Size: suitcase & day pack?
a. Suitcase: Yes, the bigger the suitcase, the more ’stuff’ you can pack in it. You should be able to carry it in a pinch, even though once at cruise ship’s check-in, crew may haul it to your room. Tip ...Yes!!!
Anecdote: Jim & Mary’s cruise luggage: On my only big cruise ship experience, my friends had 3 giant suitcases that Joe could barely lift. 
I vividly recall his overweight face, bulging red with exertion as he pushed, shoved, & dragged each one separately up 2 flights of stairs. (Note: they should have only needed one in their room, other 2 left downstairs)
b. Daypack: a small daypack for Shore Excursions essentials: medications, travel docs, etc.... for:
1) airline Carry-on and
2) 4-7 hour Shore Excursions [pplk: Safety: Soft Crime],
3) clothes change while waiting a few hours for your luggage
to be delivered to your cabin.
Remember it should be small & comfortable to carry around for few hours. You are not overnight camping. �
Anecdote: Camino de Santiago: for my 600-mile trek across north-central Spain, I purposely carried only a medium-sized day pack of essentials.
3. Number of pieces? Cruise ships usually recommend, but seldom enforce:
a. Number of Bags: suggest 1/person for 3-5 nights cruises, and 2 bags for 6 day+ cruises. KEY: is cabin space.
b. Size & Weight: more likely airline limitations will apply.
CAUTION: Overly-fashion-conscious tourist’s may sense a social need to make multiple clothing changes each day, every day, translating into “more stuff.” & maybe more suitcases.
BEST Strategy: be a Fashion Clever Traveler by designing a ”capsule wardrobe.”: See a complete explanation immediately above under the “Wardrobe” section. [jlk: Wardrobe] to/from link pair
Anecdote: MY free independent traveler’s (FIT) perspective: Resist the belief that others gauge your travel skills, character or status based on your luggage & wardrobe.
I never dwelt on others. If they dwelt on mine, it was none of my business & I didn’t care. I doubt anyone, but pickpockets do care.
4. Durability: most tech’s synthetics are essentially bombproof.
Deeper Luggage insights:
In the "Golden Age of Ocean Liners” the wealthy, as they do, packed many steamer trunks with everything their elite social status demanded, carried by servants etc. 
The metal & wood banded trunks were close to bomb-proof which is why they are a popular American antique at the ends of beds to hold blankets. They lasted.
New technology’s synthetic fabrics & plastic structure or aluminum make most luggage essentially bombproof.
MY Solo independent traveler’s perspective:
Independent travelers opt for high mobility, low volume & weight luggage because we have to transport it in a variety of vehicles, up & down stairs, long airport concourses, city streets, & rural villages, soft wheel -grabbing sand , etc.
Anecdote: daypack & medium rolling soft bag: I have traveled the world as a SIT (solo independent traveler) [jlk: SIT - below] for 20 years with the same soft, but rugged, medium size rolling bag dragging it down endless rough cobbled lanes & 3rd world dirt streets; my daypack on my back.
Over time it looked worse & worse, occasionally needing repair, but presumably a less likely target of thieves. Ironically, it was so ratty that TSA frequently ‘randomly’ (LOL) inspected it AFTER I checked it. �. (TSA leaves a little note each time.)
Oh, & my wardrobe & ‘stuff’ always fit because it was designed to.
Legendary novice tourist mistake: Too much stuff …. including me 25 years ago for several reasons:
1) Older we get, the more ‘necessities’ we require, e.g. my dozen prescript drugs & vitamins. [pplk: Health: to/from]
2) fashion-conscious tourists may sense a social mandate to ‘compete’ with other passengers’ large stylish wardrobes AND THUS, need large, multiple suitcases to carry it all.
In fact, the fashion-clever traveler creates a "capsule wardrobe,” significantly reducing the volume of stuff needing to be packed.
Nordstrom: “WHAT IS A CAPSULE WARDROBE?”
https://www.nordstrom.com/browse/content/blog/capsule-wardrobe
3) try to anticipate every possible minor & major need that might ‘possibly ‘ arise. Look closely at my Inventory list and you will see my examples of this. [pplk” Resource: INV list. to/from]
Anecdote: Skopje, Macedonia’s hostel’s dull knives: kitchen knives were too dull for salads. So, I sharpened the knife on a brick wall outside the kitchen. Thereafter, I carried a tiny sharpening stone 3” x ¾”. Tiny, but useful, to make salads easily. Also I carried a very tiny emergency can opener cuz I once needed one. pik. I
Note: If a minor crisis, your guide can help. Not their 1st rodeo. �.
Anecdote: CdeS: walking 300+ (?) miles a woman carried a separate toiletries bag with multiple forms of soap: hand, hair, body lotion, etc. I carried a couple of small hotel soap bars.
My Independent Traveler's Inventory List (of stuff):
Over time, I developed an exhaustive (obsessive) Inventory List [pplk: Resources: INV] that detailed every single item, it's quantity & its specific location in various individual compartments & pockets of both my day pack & rolling luggage.
My Inventory List was useful for several reasons:
1. I knew exactly where each item was & could fast & easily find it.
2. I could easily add necessary items or subtract unnecessary
items based on each new trip’s climate, etc. (think So Mexico
vs Antarctica)
3. I could easily amend my last Inventory List for each future trips,
A cruiser can do the same.
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