Tactics against Necromancers.

Can you explain one more thing? I read the rules, but how does Necropolis manage to gather 3-4k skeletons by the second month if everything they can use is restricted? (If possible, explain it in a simple way)

Simple enough? No problem.
You are an experienced and strong player. You probably play mostly random maps. You've probably noticed that sometimes, when a map is generated, it seems similar to another map you've played before.
I'll be frank – your feeling is absolutely correct. Maps are indeed generated according to certain patterns. These patterns are called "templates."
Through observation, players have accumulated knowledge about the generation patterns – about the templates. And it has become clear that some templates are more interesting, more "playable," well, you understand, while others are not very good.
Currently, players mainly use a small number of templates. And these templates are very rich, with many different objects.

Now, let's get to the point. A map is generated, and it has many crypts, dwarven treasure vaults, other resource storage locations (for example, where imps guard them), griffin conservatories, etc.
So, the hero goes to these structures, and there is a large number of guards, so the number of skeletons grows. For example, along with 10 angels, there are also 150 skeletons. As for the rest, you understand, everyone falls under the impact of a large stack of skeletons, whether it's 30 dwarves or 60 imps. That's how the little skeletons accumulate, the white bones...