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With moderate luck, Galtran arrives on the first day with 2 stacks, each containing 20-30 skeletons, totaling 50 skeletons, plus another 20-30 skeletons from the second native hero in the tavern. That's already 70 skeletons. We build a transformer, buy a non-native hero; here, luck plays a role: it could be Goblins, Trogolodites, or it could be Gargoyles, Gnomes, or Harpies. We transform them into skeletons, let's say around 20. That's 85-90 skeletons. Then, we buy 12 more skeletons at the fort. In total, by level 111, we have 90-110 skeletons.
Next, we focus on leveling up Necromancy, using "experience stones" and looting chests. Besides Necromancy, we can only level up Logistics. We gradually fight battles, avoiding ranged units; the best option is hordes (50-99) of first-level infantry. Friday battles are great for Necromancers! This is where you can get a lot of skeletons.
Level 121 arrives. We buy another native hero, adding 20-30 skeletons. We buy 12-18 skeletons at the fort/citadel. We buy a non-native hero and transform their units into skeletons. In total, by level 121, we have a passive income of 160-180 skeletons. And if you're good at fighting with melee units, you can get even more.
P.S. I completely forgot to mention that building a Necromancy amplifier is prohibited in online games according to the rules. So, upgraded Vampires are very rare. It's better to build Liches; it's much easier and more fun to use ranged units to mow down skeletons.
P.P.S. As for the 450 skeletons at level 121, it depends on the map and the layout. It's easier on the surface than in the Underworld. Also, you might find artifacts that boost Necromancy. On Jebus, the native stronghold has 2 villages, from which you can bring more units to the transformer, and you can quickly capture secondary strongholds.