Fewer than almost every first-time passenger expects. Expedition clothing is built to be reworn and the ship is warm and casual. Two base-layer sets, one or two midlayers, one waterproof pant, several pairs of socks and underwear, and a small onboard wardrobe covers most voyages. You do not need a fresh thermal outfit for each landing.
For shipboard life, a few interchangeable tops, two or three comfortable bottoms, sleepwear, and one optional smart-casual outfit. Pick a simple colour palette so everything layers together.
Laundry varies. Some ships have paid service, some self-service, some neither, so check yours. Merino can be aired and reworn, synthetics hand wash and dry overnight.
What people most commonly overpack is duplicate items doing the same job: several heavy sweaters, multiple bulky coats, more than one pair of heavy waterproof pants, and formal wear. Duplicate the things whose failure would stop you going ashore, meaning gloves, sunglasses, medication and charging. Don't duplicate the rest.