Manpower depends on the population count in each state a country owns and its conscription law. The harsher the conscription law and the bigger the population is the more manpower will be available.
At top left of the screen the player can see their stored manpower. Stored manpower can be spent on creating and reinforcing divisions.
Manpower and divisions
Manpower is needed for every division, the exact number needed depends on the makeup of the division.
Manpower can be lost as casualties of war as well as to attrition. Divisions that lack full manpower will be reinforced from the manpower storage.