Currently in DF when you read a necromancer book you become a master necromancer that can instantly go to a catacombs & raise an army capable of whipping out the world. Instead have a skill, when you read a necro book you become a novice. All other skill comes from raising zombies or taking control of zombies.
Novice: Undead don't attack you and you can raise zombies, zombies follow you when created. Limit to 5 zombies
Adequate: tell zombies to not follow you, at any time you can tell them to follow you, they still don't follow other necromancers orders. 10 zombies
competent: 15 zombies
Skilled: tell zombies to actively guard a site from people hostile to you 20 zombies
Proficient: 25 zombies
Talented: tell zombies to not kill living things that are not hostile to you or kill things that are undead 30 zombies
Adept: 35 zombies
Expert: tell zombies to takeover a specific site without you coming with them 40 zombies
Great: 45 zombies
accomplished: take control of a other necromancer's zombie(s), the higher your skill the better the chance of succeeding and skill of the owner lowers the chance of succeeding 50 zombies
Master: take control of a wild thrall, low chance, failing makes the thrall hostile to you & not able to be captured, the more skill you have past master gives you a higher chance 60 zombies
High master: 65 zombies
Grand master: no limit to amount of zombies