If you happen to die at a boss. You will have no corpses anywhere to summon new minions.
EDIT: Why should necromancers be punished extra much from dying? Do other classes have the same issues? I think not…
Well, there are some skills that grant you some chance to create a corpse on doing damage (up to 12%, doubled for bosses). 
Yes I am aware. But going in with full minions and zero is quite a difference regardless.
i think theres a golem ability that sacrifices itself to resurrect the skeletons
but i have found that you can run out of corpses, yeah
Ive noticed the same too. I can go into a boss with full minions with the talent for 12% increased (before doubled) for bosses and taking decompose which specifically states “creates a usable corpse every 2.5 seconds”. Yet on boss fights without minions that spawn I generate little to no corpses on bosses which is kind of tedious and bothersome considering we have talents specifically that state increase chances on bosses. Yet no corpses are made I feel this is a huge oversight.