SandStone,
As I said in reply #1, each land trade city tries to keep a certain number of caravans on the map. The maximum number of caravans appears to be the average number of dots (or shields) for all its tradable goods on the world map, rounded up. In other words, a land trade city will try to have 1 caravan on the map if all normal (without Ra's "trade more" blessing) trade quotas are 1500, 3 caravans if the number of normal trade quotas that are 4000 is greater than the number of normal trade quotas that are 1500, and 2 caravans otherwise.
The number of trade caravans per year depends on how long each caravan remains on the map. In quite a few missions, a land trade city is not able to trade all desired goods. For example, in Meidum, it would be nice if Saqqara bought 2500 wood, 2500 bricks, 2500 pottery, and 2500 papyrus each year, but the long distance between the entry and exit points makes that impossible (unless you kill the caravans after they trade). For another example, in Dakhla Oasis, it is similarly impossible for On to sell 4000 reeds and 2500 meat each year. On the other hand, if the entry and exit points are fairly close together, as in Buhen and Iunet, then it's possible for a lot of caravans to visit each year.
[This message has been edited by Brugle (edited 06-30-2003 @ 06:45 PM).]