Demis Celdren:
How many players do you want minimum?
Sorry, missed this.
Crew for a ship is one person per action per weapon (For instance, a Ballista takes 3 actions: load, aim, fire, so 3 people to fire it in one round), plus a captain and a helmsman.
That's air supply and full combat ability. Presumably the Captain is for coordinating things and making final decisions, but, realistically, the helmsman decides where the ship goes, and the trigger man decides when to fire, assuming it got loaded and aimed. We will be assuming folks do what the Captain designate says unless they tell me otherwise specifically.
An Artificer can reduce man power by one loader with the right Infusion. A Battlesmith can further use his Defender as the trigger man, letting him man a ballista alone (with a little preparation).
And you were expecting a simple answers.
So, given the ship I plan on the Patron group providing, you'll want 6 in the party. If you DO have an Artificer, that could free up a sorcerer or wizard to cast spells, but that's usually not an issue due to range.
I will close things out at 8, just to have a body or two to absorb inevitable losses. Assuming we get to 8.
Four is a bare minimum: one fully crewed Ballista and a helmsman, but that makes repelling boarders or doing your own boarding a bit iffy.