From polygon's preview
Also, Mirror Knight can summon more than one minion in the boss fight
The game also appears to feature the return of a Demon's Souls gameplay mechanic: Black Phantoms of run of the mill enemies — as opposed to named Black Phantoms, such as Maneater Mildred — can appear out of thin air. In one section of the demo, a trio of Black Phantoms materialized, tossing two dreglings and an armored tortoise knight at the player at once.
Also, Mirror Knight can summon more than one minion in the boss fight
http://www.polygon.com/2013/6/11/4417986/dark-souls-2-e3-previewMirror Knight can summon a second ally through his shield and in the case of our encounter with the boss, called forth another knight with a different weapon. It's not clear just how many knights Mirror Knight can spawn or if their weapons are randomized.