In thinking about supply wagons and supply you need to think at the element level in the game. Elements are the sub-units in the game. If you click on a unit the display on the bottom right should show one to four elements.
Supply wagons can hold up to four elements. Each element can hold up to 80 points. Each point can roughly supply one element of troops. So a normal army can live two months off of Inherent Supply the troops carry, and then, extend this by the supply wagons.
A small army will have around 50 elements, a medium army around 100 elements. So a couple of wagons shuttling back and forth ought to be able to keep these forces in supply.
When you think about depots, these are not just storage. They create supply each turn which wagons can then collect, just as many cities do.
Elements are also a way of thinking about defending inside fortresses. Counting elements and supply, remember the fortress is not generating supply is besieged, you can determine if a supply wagon would be useful in extending the defense of the city.