What determines the number of peasants/militia/e-militia that will fight in an battle.

I've been going through some battle reports that I have saved on the board. And I can see a variety of things.

Once there were 10 peasants defending , 2-3 times there were 15 , another time there were 18. (without militia or anything)

With militia and e-militia the number grows from 25-32 so I'm asking what is the factor that determines how many will defend the field. Is it a random factor or is it influenced with fiefdoms on the field etc.

thank you for your answers.