Your armies are tied directly to your POPs. Basically, you should always as many active units as possible. A soldier POP gives one recruitable unit per 3000 people.
When in battle, your units take damage, which directly affects your POPs as well (with a certain factor). So losing an X amount of men decreases the size of that POP by Y amount. Most of the time, you won't be recruiting any new units in a war, unless some of your armies get completely destroyed (ie. removed from the map). And that is only in the case where the POP didn't take enough damage to not supply you with units anymore.
Your units are reinforced based on three things: Supply, attrition and POP size. Your supply, or your military funding slider (the one that pays for all the equipment) sets the maximum amount of troops your units will reinforce. Attrition is caused by being in hostile territory or over the unit limit of a province. It both kills some of your soldiers (though this does not affect your POPs afaik) and it slows down your reinforce rate.
A POP that isn't completely "full" will also slow down your reinforcement. So if you recruit a soldier, and then the size of the POP drops from 3000 to, say, 2000 due to battle, you'll notice the icon of the recruited unit turning yellow. This means that the POP is too small to reinforce the unit at full speed. At very small POP size, the icon will turn red, which means that reinforcing is going to take ages. It is quite important that you try to keep your soldier POPs at full strength when possible. I usually try to have a 2%-5% nation wide population of soldiers, and during war I maximise my soldier/officer wages to max and put all my national focuses to encourage soldiers.