Finger of Death is on the Sorcerer, Wizard and Warlock Spell list. That could be used to create a new zombie a few times a day that doesn’t need to be reasserted. So technically, give the party a year and they’ll have 2,190 permanent zombies, not including the ones created with Animate Dead.
It would be interesting to play a campaign of necromancers with the goal of taking over the world. Searching for valuable artifacts that increase their power, dodging Paladin and adventurous parties. Building their undead army one soldier at a time.
Unfortunately, I’d much rather play such a game than run it.