Necro's have a standard pet which is always around. Then there is wake the dead, which raises one pet from one corpse. Also there is Army of the dead which raises 3 pets at level 1 it has three levels which add 1 pet each. Army of the dead and wake the dead can not be used at the same time. Then there is swarm of decay which adds archers one per level up to three. Then there is the epic pet a big red skeliton. Also there is suspend minion, only that pet is in a pocket and not seen.
So in a battle a necro with all the AA's maxed could have 11 pets up at once as follows;
1 standard pet
6 army of the dead
3 swarm of decay
1 epic pet
Yeah necros are cool. Remember that the army of the dead, swarm of decay and epic pet despawn rather quickly and are dumb pets that you can't command to attack.