There are plenty of threads about it if you look around.. it's not about how many ships you have, its all about trade power. Keep sending ships until you reach at least 50%, more if the node is rich enough

Just remember that there is severely diminishing returns at higher levels of trade power.. getting from 80 to 90% trade power in a node probably isn't going to be cost effective, but 50 to 60% might be...
Also, propagating trade from downstream trade nodes is a major factor, so it's usually worth it to stack trade power in your home node until you have a clear majority, just make sure to have neighboring nodes filled with merchants directing trade to you.
Thirdly, it's not necessary to keep a merchant in the channel, he only provides +20% bonus to income in your home node. It's very possible you will get more from sending him elsewhere, particularly early-mid game.
Edit: The AI is terrible at trading too.. later in the game, Sevilla at least will almost certainly be immensely rich simply by virtue of being a colonial end-node. It's almost always worth it to send a bunch of ships there on a collect mission, at least once it hits 100 value or so.