Questions about the Heroes 4 map editor.

Here, you can set up a repeating event that checks a condition:
"IF the number of peasants > 10, then the number of peasants = 10" (or something similar). The condition is checked every day.

You are right, we set a calendar event on a neutral creature with a repetition every day, then we set a condition - if the total number of creatures in the army (peasants) is greater than 10, then subtract 1 peasant.
Thus, there will be no more than 10 peasants in this army.