There isn't an ideal. I usually have a convoy doing rounds around the central region (Luebeck, Rostock, Sttetin, Gdansk, Malmo and Alborg). The game will tell you how long the convoy will need to make a round trip. During the time of the round trip, ideally no city should run out of goods, so your convoy should have enough goods to supply all the cities along its route, at least with the goods that you produce yourself. From this you should be able to calculate how many ships you need.
A more ad hoc approach is to simply build extra ships and increase the amount of goods the convoy carries until you see that the cities are not running out.