So, a friend and I were chatting about necromancy in D&D and I got off onto a tangent about leadership/undead leadership, creating your own necropolitans, and so on. Not really relative to the matter at hand, but that's where my thought processes took over and led me down this path.
A Dread Necromancer 20 animates 4 + CHA mod per caster level of undead.
-Assuming 18 CHA to begin, +5 inherent, +5 level, +6 item, we reach a 34 (+12) by level 20, making this 16 HD per caster level, or [(4+12)*20], or 320 HD of undead.
Desecrate doubles the base number (4) to (8), making this [(8+12)*20], or 400HD of undead.
Rod of Undead Mastery (Libris Mortis) doubles this base number again, but with the way the multiplicatives work in D&D, it triples it instead, making the base number (4) into (12), so it becomes (12+12)*20, or 480HD of undead.
Stitched Flesh Familiar adds +4 HD of undead - whether this modifies the base amount (4) to make (8) or simply adds post the multiplication process (12 to 16) changes the numbers considerably - though its reading suggests the latter. The numbers then become either 720HD (under the former, [(24+12)*20]) or 560HD (under the latter, [(12+16)*20) - both respectable numbers.
So... is my math correct? Barring GCF abuse (which would lead to some astronomical numbers, surely), it's either 720 or 560 HD total that can be raised in a single casting of animate dead.