Unless you're a necromancer, or something like the Geth in Mass Effect, there is more to raising an army than just building troops. And if you're running a game where things are getting political, it can be good to know what types of armies may be in play. After all, politics can turn to war very quickly. So today I want to talk about a few considerations to keep in mind when looking to building armies for your game.
Scope & Scale Matters
Flat out, if you're running an Epic Fantasy type game, or a Space Opera type game, you have a lot more room to make armies absolutely huge. The whole thing with both genres is things get larger than life. Armies numbering in the hundreds of thousands can be raised and do battle without as much concern for food, training, and cost as in more grounded settings.
A large battle in a gritty dark fantasy might just be 50 people on a side going at it. That same large battle moved to Epic Fantasy could be forces of dozens of thousands of people. See the difference?
The Costs of an Army
An army ultimately costs three things: Food, Money, and Time.
Food because you have to feed your army. Money because you have to equip and train your army. Time because you have to train your army.
An area's ability to do those three things will greatly influence how large an army it can raise. At the same time, the larger the army gets the more Food and Money you need, so the kingdom needs to sustain that as well. In World War 2 in the real world, this was done by moving women into the work force in typically male dominated jobs, while the men were off fighting. This can work in your game too, but it does come with the built in word detail that men are expected to fight for their country, while women are not. If you have a more egalitarian world, then just remember that someone has to keep working the farms and making the weapons/armor.
That said, these three costs are also worth keeping in mind for another reason. That reason being...
Standing vs. Raisable Troops
How big an army does a place have standing, versus how many could it have given time? In peace times you don't need as large a force. And having a smaller force also reduces the costs of food and money. However, once war is going on - or looks like it could be - you need to bring in those other troops. Reserves get activated. People get asked to come out of retirement. Recruiting drives go on. Drafts happen.
So don't just consider how many troops a place has now, but keep in mind how many they could have given time (and how much time.) It is very possible that a place with a smaller standing force could explode into the largest army in play if given the time to pull everyone in and put them in position. It is also possible, and even likely, that those with larger standing armies can't grow as fast because they have more people already in the army.
Size Isn't Everything
Worth keeping in mind, but size isn't everything. Fantasy/Sci-Fi worlds are full of claims like 1 sardaukar form Dune is worth 5 men from another house's army, and sometimes that is a boast, but sometimes it is just a fact as well. It is possible for one group to have a smaller numerical army, that is overall stronger because of the quality of training. It is even a good way to explain certain groups being so strong militarily despite smaller land area.
However, attrition tends to affect those forces more than other armies. After all....
If war does happen it is good to remember that resources tend to deplete over time with a war. It is harder to feed and pay for an army as food and money is used - or lost. Taking those resources from the enemy can be a good strategy, but is also hard. However, time is the thing that is hard to get back. The longer a war goes on, the more the need for fresh troops, the less time you'll see given to training.
A six month training program in peace time could be reduced to a six week course - or even further - in the heart of a war. Actual combat experience then is the teacher, but starting off...troops are less trained.
Abstract Works Best
I'm a big fan of using abstract for big battles and military positioning. I'm not a strategist. I'm not even very good at the RTS genre of games. Characters can be, and that can be handled through more abstract means. Still, even when dealing with the abstract it is good to understand some of the - heavily simplified - core considerations. If only for consistency's sake.