Speakeasy
How many barrels can I buy from a Rum-runner ship, and when does the ship move?
The designer answered this directly in the official FAQ or a forum reply.
At most two, enough to fill one Truck. As soon as you finish buying, the ship moves clockwise to the next empty port, so a second Truck cannot buy from it at the same port, and you cannot buy extra for buildings in that district. If a Truck later reaches the ship at its new port you can buy there. The price is the number under the rightmost barrel.