Is there a maximum of light ships on one trade route?

[GAGA Extrem]

Or you can place all your ships on one route to gain maximal trade power?

You can use an unlimited number of ships to protect a trade node. This can indeed be a good strategy if it is one of the major New World nodes (Chesapeake Bay / Carribean).
But remember that trade power from ships does only apply to that node alone, so there is no reason to pile more ship on it if you already have 95% of the total trade power there.