Animate Dead lets you raise 1 undead, and requires you to use the spell to reassert control over it every 24 hours. When cast with a spell slot of 4th level or higher, you can raise/reassert control over 2 additional undead for each slot level above 3rd.
- 3rd Level Slot: 1 undead at a time
- 4th Level Slot: 3 undead at a time
- 5th Level Slot: 5 undead at a time
- 6th Level Slot: 7 undead at a time
- 7th Level Slot: 9 undead at a time
- 8th Level Slot: 11 undead at a time
- 9th Level Slot: 13 undead at a time
Animate Dead is available to Wizards, Clerics, Circle of Spores Druids, and Oathbreaker Paladins. The first 3 all have the full number of spell slots, while Oathbreakers have less.
If we assume that CR is perfectly balanced, a full caster can multiply their power by using Animate Dead. A level 20 caster can create CR20 worth of zombies, incredible! However zombies have only 8AC, 22HP, +3 hit, 4 damage, 20ft speed, while skeletons have 13AC, 13HP, +4 hit, 5 damage, 30ft speed. Spending all your spell slots to maintain a shambling horde may not be an effective use of your abilities.