I'm looking at the PHB now. I see the spell description as indicating one creature when cast at 3rd level, +two creatures per higher-level slot. Necromancer (wizard specialist) adds one creature per casting. I see that as being two creatures from a 3rd-level slot, four from a 4th-level slot. I'm willing to be proven wrong, though.
Just so I can check my work against yours, how are you getting to your numbers?