Population Cap

Population capacity granted by constructing houses
To change population capacity granted by constructing houses, load a tuning pack mod and navigate to statemodel_schema/buildings/house. Clone it without changing the name. Now go to schema_bag/int_properties. There you will see just one modifiable property with int_property_id of population_cap_increase and value of 10. Change the value to the one you want and you can proceed to test your mod.

Maximum population capacity
Changing maximum population capacity can be done through both script and tuning packs.

Always check the scardoc for full related functionality. Here are some examples of the more relevant functions:

To change maximum population using a tuning pack load a tuning pack mod and navigate to army attribute editor. You will have to clone every separate civ file you want to modify. and go to army_bag/race_population_cap_table. The value you want to adjust here is max_personnel_cap. Change the value to the one you want and you can proceed to test your mod.