Consider a typical galleon used for cargo around the 18th century.
How would the number and roles of the crew differ between a normal, full crew, and a "skeleton" crew, on a voyage of a few weeks?
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Galleons were less common in the 18th Century than they were in their heyday, a century or two before. Merchant vessels typically didn't carry much in the way of 'spare' crew because it didn't make much economic sense to carry people who weren't essential to the ship's operations.
The exact number of crew would be dependant on the size and nature of the vessel, the amount of sail carried, the number of guns carried and whether it was likely to need those guns (i.e. was it likely to encounter combat). A Spanish treasure galleon would be larger, carry more guns and therefore have a much larger crew than a galleon carrying manufactured goods around Europe.
In peacetime, a skeleton crew could be the Captain (who would also serve as the ship's sailing master), a helmsman, sufficient seamen to work the sails, and possibly a few petty officers to oversee those sailors. For longer voyages, you'd also include a ship's carpenter and a sailmaker to aid repairs on the voyage.
In wartime, it would carry additional men to serve the guns. You'd need an experienced gunner for each gun and additional men to load and run out the guns. The cannon of a merchant vessel were usually smaller, lighter and less powerful than their military equivalents. They, therefore, could manage with a smaller guncrew to work them (often half the number of an equivalent weapon on a naval ship). Possibly, you'd also have a ship's surgeon, if the voyage was considered risky enough.