How to get more Manpower

Barracks increase manpower by a percentage, up to a 50% bonus. Constructing them in your highest manpower provinces will have the greatest effect.

... And if you like to try a country with a lot of manpower, try India in the 1939 map ... Quite a contrast to South Africa in the same map ...

Manpower comes from your population.

your population eats food and consumes goods.

This happens to be a 1+1 = 1 ratio (+cash)

1 food + 1 good = 1 manpower.

When you conquer enemy lands, you will see you have to feed all the people you conquer. The food that comes from the countries you conquer about evens out with the food needed the non-food producing countries consume. (at least on the 22 and 10 player europe maps) So the end result is that when you conquer lands, your net food remains about equal while your manpower rises.

The second method as indicated is barracks. Barracks also consume food and produce menpower. Problem is, the conversion ratio is worse than from land unless you have cities that produce about 2000 menpower unimproved. And you are not conquering food producing provinces to make up for the food loss. Therefore, you want to conquer land if possible and use barracks only in emergencies. In fact, its rather important to turn them all off when you conquer any.

The third is to change your unit composition. Be aware that the upkeep of your units is what truly matters, moreso than their production cost. You are limited in army size by your upkeep and in my experience, you can touch those limits relatively quickly if you fight with little losses, dont fight at all and/or dont expand much.

Considering food and menpower are essentialy the same thing, all infrantry units consume a lot of that. I am no veteran yet after 5 games, but what i have learned in those 5 games is that not only should you never build militia, i don't build infantry either. I upgrade them once or twice, then let them fade out while only using artillery and AA from the infantry class with tanks providing the hitpoints. Maybe that might change if i ever get into games with all very active players that build and micromanage stacks properly, but as long as its about stomping noobs and AI's on the 22/10 player europe maps, infantry is a waste of food/menpower. Except artillery of course artillery is super mega awesome against AI and people who are offline for multiple hours most of the time.