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 an unspecified level, in a section describing a particular class feature (though within the Combat section, I realize), as referring to the class level of the class that gives it to you. It would have to say “character level” to specify all of your HD. (But yes, Racial Hit Dice most certainly do apply towards your character level; LA does not, however.)

Thus,

If you would turn, but your cleric level is twice the creature’s HD, you destroy

Destroying doesn’t come into play when determining which undead you turn. You roll your turning check to determine your limit on how powerful an undead you can turn, and then roll your turning damage to determine how many HD, total, you actually do turn. Then each undead in the area, starting with those closest to you and ignoring any that exceed the HD total determined by your turning check, are turned. If any of those undead you would turn have less than half your cleric level in HD, they are instead destroyed.