The tooltip for this skill says “you are limited to 2 mages at a time”, but when I first unlocked this skill and put it on the bar (no specialization points allocated) I was able to summon 3 mages.
Later when I selected the specialization node that gives you an additional mage (Order of Death), I could still only summon 3.
Did you pick Necro for you mastery? That gives you +1 skellie mage, when I take the skill as a Lich I can only summon 2 at a time. When I swapped over to my Necro I was able to summon 3 by default & 4 with Order of Death.