Again, lets take a look at Mount & Blade. You have these places on the map, spots that spawn a pre defined number of groups belonging to a certain faction. So for example you have a faction called “looters” and they have a XY coordinates place on the map where they spawn. If there are fewer than 36 looters groups on the map, every 72 game hours a group gets spawned. Each group can also have multiple unit types all spawned randomly between numbers X and Y (you can also set minimum number of each type that is guaranteed to spawn).
There’s already a formula sort of like this:
Enemy camps will spawn onto the map from time to time, especially when you’re wiping them all out it seems. When they do, they’ll start with a finite number of resources, but will generate more resources by raiding caravans and villages and so forth. Every victory = resources for the camp. They use those resources to generate more troops which in turn get them more resources.
A big orc camp, for instance, will probably have a couple hunting parties, a large base defense, and a massive raiding party that tromps around toward villages and burns everything in sight. Each time one of those groups returns, it gives resources to the camp, and more dudes can be spawned.
Again, the system is there. You just haven’t seen it yet.