Invocation of Nehek (Vampire Counts 8th Ed, pg 60) says that "Wounds gained are distributed as described in Ressurecting Fallen Warriors".
The rules for Ressurecting Fallen Warriors (Vampire Counts 8th Ed, pg 26) state that "A unit cannot be taken beyond it's starting size, though certain powers, spells and special rules may provide exceptions".
So the unit will not go above it's starting size unless the unit or the caster has a special rule saying they may do so (Swarms do not).
The rules for Ressurecting Fallen Warriors go on to say:
"In the case of multiple Wound models, all models in the unit must be fully healed before another can be resurrected."
However, Invocation of Nehek does add that "models with the Vampiric, Ethereal or Large Target special rules can never regain more than 1 Wound per successful casting."