Those wishing to write war stories often struggle to determine how many troops are in a settlement, sovereign power, or in an army facing another army. This will depend partly on things like forced military training, expectations, the level of threat in recent decades and today, and even the sort of government that exists. This allows some leeway, but a military junta is likely to have a high per-capita number of available soldiers compared to a monarchy, which might count farmers in its ranks, as opposed to trained warriors.
For perspective, the United States has almost 7 soldiers (combined military branches) per 1,000 people, or nearly 7 per 1000 capita. A city of one million can therefore produce 7,000 soldiers. It’s not quite that simple, of course. Army, Navy, and Air Force bases create concentrations of personnel where those bases are located. But the soldiers may have joined the military from a broad area. Some cities or regions will contribute more or less than others, partly for cultural reasons.
We don’t need to get this detailed, and we can fudge the per capita higher or lower based on our needs, with no one able to argue with us. This number varies considerably across Earth nations, from about 2 per 1000 capita to over 100, with many below 10. We can choose a typical per capita and assume this as a default. Then state in each settlement or sovereign power file whether these armed forces produced from here are at, below, or above the typical per capita. If we ever need a war and want troop numbers, we can add the total population of cities and towns in a power and arrive at a likely army size.
Formula: Population * Per capita = Troops
Example: 1,000,000 * .007 = 7,000
This becomes useful when determining who has the bigger army. We can change the numbers if we want Kingdom 1 or Kingdom 2 to have the larger number of troops. There are more towns and settlements on our map than we’ve likely drawn, so we can also raise the number upward by 10-20% for all of those unaccounted for settlements. Recent battles or famine are just two of many scenarios that could’ve reduced one power’s available forces. Such details add believability and are arguably better than just deciding one army is bigger than the other because we said so.