My first naive approach is:
- Given N number of salesmen. Cluster an area to be N clusters with almost equal number of clients  with some kind of same-size clustering/facility location algorithm
- Assign 1 salesman to 1 territory (one-to-one assignment)
The question is:
- How should we divide the territory gracefully if the number of salesmen increase/decrease from N=10 to N=9, N=11, or N=20?
- Is one-to-one assignment is doomed to fail? What are the alternatives for designing and scaling sales territory?
- the location of centroids could be very different between N=9, N=10, N=11. It can make the territories very different.
- increase of salesman from N=10 to N=20 will shrink the area of previous territory
Out of scope for this question:
- Routing (TSP/VRP). TSP with Time Window would be done after sales territory, but it's out of this question scope. This question is more about designing/scaling the sales territory itself.
 We could use equal expected sales amount, but for argument sake let's use number of clients.