@Unternehmer So I have started a map with Man Power as a key component to the design. Here is how I addressed it.
The cities generate Man Power (amongst other things). So each nation is given cities based on what I feel the over all manpower should be.
Then as you loose territories with cities, your ability to generate new armies becomes more difficult. On the flip side captured cities provide fewer resources (including Man Power) to the occupier.
What I have done is provide each nation with a stockpile of Man power at the start of the game relative to what they were capable of historically and what is generated each turn with cities will not even cover the average expenditures for a nation each turn. Thus on the timeline of a game... nations without vast starting reserves will have to be very conscious of their Man Power expenditures each turn.