You can have a full stack on each node. The first one gives 20 units of trade goods. Every next ship will add 13 extra units of trade goods to the trade lane. The more ships you place on a node the higher your supply of that good becomes. Higher supply means lower prices. Just don't place to many ships at once and slowly increase the amount until prices start to drop. Then you have to stop until prices are rising again or start stabilizing.

There is a bug of course. If you want to remove at least one ship of a stack, remove the stack of the node and split the stack. Next place the remaining ships back on the stack again. If you split a stack while on a node, the remaining ships will become stuck and become useless.