General:
* Population density (if focusing on the number of people per unit area)
* Population spread
* Population pattern
* Population geography
* Spatial distribution of population
* Population dispersion
* Population arrangement
* Population structure (if focusing on age, sex, etc.)
More specific:
* Urbanization (if focusing on the movement of people to cities)
* Migration patterns (if focusing on movement of people over time)
* Settlement patterns (if focusing on how people are clustered in space)
The best synonym will depend on the specific meaning you want to convey.