Questions about the Heroes 4 map editor.

Next, we set a condition: if the number of creatures (peasants) is greater than 10, then we subtract 1 peasant. And click OK. Now, the number of peasants will not exceed 10.

I think I've found a solution: just set a repeating event for the peasants: every day, all peasants will be expelled, and 10 will be added.
However, I haven't tested this method.
If it doesn't work, you can use this trick:
1. Give 1 leprechaun.
2. Expel all peasants.
3. Give 10 peasants.
4. Expel the leprechaun.

This should work 100%.