Cohorts. Lots of necromancer cohorts.
How do you get multiple cohorts, you ask? Simple, I say.
Get one with the Leadership feat.
Pick up the Pale Master PrC, and get an undead vassal -- go for one of the sentient types, like a Vampire, and make sure he has levels of cleric or wizard (as preferred).
Pick up the Dragon Cohort feat (from Draconomicon), get a wee bairn of a chromatic dragon, and make sure he has Sorceror levels, and knows the Animate Dead spell.
OR maybe, depending on the necromancer in question ... I've been told of a Prestige Class, I forget which book it's from, called Dread Master or the like. Lots and lots and LOTS of cohorts.
If they were all, say ... clerics of the deity of Undeath and Carnage that you, yourself, are a high priest(ess) of ... hehehe ...