Questions about the Heroes 4 map editor.

Tell me, how do I implement a simple task like limiting the increase in the number of neutral creatures? For example, if the default is 10 peasants, how do I prevent them from becoming, say, 40 after 2 weeks?

Try setting a recurring calendar script on the monster (e.g., every 2 weeks, "remove creatures/banish warriors" 10 at a time from "this army" – although I'm not sure if this will work with neutrals).
VARIANT #2 – before attacking, write a script: banish XXX creatures from the enemy army (this will help limit them during combat).