THE RULEBOOK REFERENCE
This stat is included for general reference. It only comes into play in one situation: When a ruler is forced to conscript all available peasants for defense of the kingdom. Gods forbid, but anything can happen.
The typical medieval European country (such as France around 1200 AD*) had a population of about 5 million. We'll use that as the base-line population figure.
Sparse 1 million
Average 5 million
Dense 10 million
Here is the formula for a kingdom with an average population of - 5 million people
Less than half of the inhabitants are male = 2 million males
Of the males, approximately 10% are between 15 and 50 years of age = 200,000 men
Of those, no more than half could be organized =
Total 100,000 men
available to fight
Remember, there's no postal service or social security numbers and many would simply refuse to take arms.
At this level of mobilization, agriculture stops, trade and commerce are interrupted, and no taxes are collected until the populace goes back to work. In cultures where women are traditionally willing and able to fight, you can add a quarter of the number (for a grand total of 125,000 able bodies). These people are sword fodder, likely to break and run in battle and suffer heavy casualties against trained troops.
The more likely scenario would be the conscription of a local rural area, city or town. Those numbers are addressed in RULES OF WAR, under RAISING ARMIES.
* Internet Medieval Sourcebook