Hi
@Carlosa,
The Mark 99 Illumination are engage only a single target at a time, but it's only used for terminal guidance, or in other words "last second engagement". Multiple missiles can be airborne at any one time and the Mk 99 will be able to engage multiple targets near simultaneously because it doesn't need to illuminate a target except for when a missile enters terminal range. Because these times are very short, terminal range exists for less than a second on Navy missiles, the Mk 99 can engage many targets within a short time-frame.
This doesn't mean the AEGIS system can only engage one target or vector one missile, just that the Mk 99 can only illuminate one at a time, but that illumination time is really short and another target can be illuminated within instants of the previous.
Does this answer your question?
Source:
Part of it is from my experience, the other is from
NAVAL RADARS (Military Weapons)