Two or more vents is ideal for intake and exhaust, but adds to the cost and isn't really necessary for a smaller boat. If you only use one vent, use the exhaust piece (they provide both intake and exhaust blades), which will draw air out of the boat, and there are enough openings with companionship doors, passive vents, and other air leaks to replace the exhausted air. Works great! They say it exchanges 800 cubic feet of air per hour, if I remember correctly.
Frank