I could get by with two, possibly--walking shoes and sandals that are dressy enought for the dining room, but usually take 4. Partly because my feet are small and they don't take up much room, but mostly because I have learned my lesson, especially with sandals, of having shoes that start out fine and can end up a day with blisters or other pain. Even when broken in.
So, I always wear some kind of walking schoes--sketchers or tie--at least two sandals for day wear (assuming a warm weather cruise) so I can switch them out if one is hurting, and at least one pear of dressier wedges for evening.
If I have room I will through in water shoes or another pair of wedges.