User blog comment:Niohoggr/NPC's character construction/@comment-29248623-20170908035042

As for NPC creation, there is a likelihood of the requirement of special components for the creation of Custom NPCs. If you remember the guild discussion concerning the Caloric Stone and the proposal of making a golem from it, that should prove as one prime hint.

I also bet that the dungeon's base nature has a limiting factor to NPC creation and there is also the matter of the "memory space" that allows for a distinct number of NPCs to inhabit an area. Niggredo's domain and Ainz comment about the funds Tabula Smaragdina put into making her area is a hint for this.