How do I create a lot of skeletons?

Yes, I'm talking about the start of the game, but with expert Necromancy and a buff from 30 Goblins, I want to get 12 skeletons, but the computer calculates it unfairly and only gives me 4-5, and it's like that every time in every battle.
I'll try again today and let you know, but there's some kind of trickery here.