[RPG] How many undead of each type can you control at one time

You don’t get separate animate dead control pools from cleric and wizard, so you only get 32 HD, not 64. Undead from Rebuke Undead are in addition to this.

From the Revised Necromancer Handbook:

You are not going to play a True Necromancer!

A lot of people love the True Necromancer, even though it’s a completely crippled class. Even a Mystic Theurge is better, and that’s saying quite a bit because that class is a dog with fleas. You’re 5 real caster levels behind the curve. If you just took Leadership, and then your cohort took Leadership, both of the cohorts would have better casting than you (being 2 levels behind and 4 levels behind respectively). You can provide the party better and more powerful Necromancy as a single classed Fighter that happens to have Leadership than you would if you were a “True” Necromancer.

Top Ten Reasons True Necromancers Are Bad

1. At 14th level, you are five caster levels behind in both classes, so if the party Fighter took Leadership, and his cohort got Leadership, he’d actually be bringing more Necromancy to the table than you. As a fighter.

2. You have to take the Death Domain as a Necromancer Cleric, which is a waste of a Domain Slot when you are trying to be good at Necromancy.

3. In the early levels, you postpone your access to Animate Dead by 4 levels.

4. At 8th level a True Necromancer can create, but not control Ghouls. A Cleric at that level can control but not create Ghouls. Guess which is better? At 11th level, the True Necromancer gets the ability to control Ghouls, and the Cleric gets the ability to create them, so there’s no point at which this is advantageous.

5. The only unique ability of the True Necromancer class is unimpressive. Desecrate is a great spell, but it’s also a second level spell.

6. True Necromancers eventually get a bonus to Rebuking – at 17th level they have a +1 bonus to their Rebuking level. But at 7th level they have a 3 level penalty to their Rebuking level. So at low levels when rebuking is good they can’t use it, and at high levels when Rebuking doesn’t matter they don’t care.

7. True Necromancers are always going to have underwhelming Save DCs. Between MAD and the fact that they are often forced to use spells that are 3 spell levels lower than what the single-classed casters can use, they’re going to be out enough Save DC that it shows. A lot.

8. As a True Necromancer you have all the disadvantages of both a Cleric (the gods can take away all your spellcasting at any time), and a Wizard (you have Arcane Spell Failure, preventing you from wearing good armor). Also, your BAB and HPs stink when compared to a Cleric.

9. Control pools from Animate Dead actually don’t accumulate between your two classes. It’ right in the spell, if you cast the spell it considers all undead you control from all castings of Animate Dead, not just your Arcane or just your Divine castings of the spell. Some people say differently, and some even quote CustServ, but when was the last time you won an argument with your DM using the line "some guy on a board said that CustServ told him....."?

10. There is almost no synergy between Cleric and Wizard Necromancy. Any synergy you desperately want to find could be replicated by just taking the Apprentice feat at first level and having some Use Magic Device. Get yourself a couple of Wizard Scrolls or something. It’s a better buy than setting 5 caster levels on fire. Smart cookies can even get the right spell effects off monsters for free, no less.

Ultimately, even if you argue that the pools are separate and you get 64 HD worth, the loss of Caster Level makes that a wash compared to simply taking the Deathbound Domain (which gives 50% more rather than 100% more, but at far lower cost).

I strongly recommend either cleric or dread necromancer for the goal of having a big army of undead minions. Wizards can work too; they won’t be as good as cleric or dread necromancer for this purpose, but they’ll be a whole lot better than true necromancers.

I see two broad options, one of which is a bit of a frame-change.

First, the frame change: most undead - especially low-level undead like zombies and skeletons - were mindless in previous editions of D&D and aren't terribly bright in 5E. It shouldn't be too hard for some passably persuasive denizens to convince the undead hordes to follow their commands without magic. Further, zombies and skeletons are frequently portrayed as basically milling about doing nothing in particular if there's nothing better to do; it shouldn't be too hard for the denizens to have corralled the undead into holding areas for later use (it's not like they need food or exercise, after all).

Option 1b, I guess, would be that there's no particular reason for the undead creatures to stop doing whatever it was that they were doing when they're no longer controlled, especially if what they were doing is a mindless, repetitive task ("keep turning this wheel" or "walk along this round pathway (ie., patrol)").

If that's not quite sufficient, the denizens could be keeping a handful of key undead creatures under their control, and trusting that the rest of them will just kinda follow along.

PCs could, in principle, do the same thing: control or convince (or just lure) the undead to where you need them to be, then let them hang out there 'til they're needed. If necessary, keep a small number under control for specific purposes.

The second option would be to adjust (slightly) the Hallow spell, and "Un-Hallow" the area (which may indicate that a more powerful creature is behind this, and the adventure is prelude). As written, Hallow appears designed for Good PCs to use; I don't see why its mirror shouldn't exist, which would allow for necromancers to exert some control over more undead than normal (possibly only just "some" control, though: perhaps it's just enough that the undead see the other denizens as "friendly" as opposed to "lunch").

Again, in principle a PC could do this, if they could find a deity willing to allow it.