Human/Dwarven towers hold BOTH human and dwarven necromancers. It simply depends on who is the first historical figure to find the local tablet holding the mysteries of life and death. The diety can be dwarven or human, this determines which race find the tablet, I believe.
This first necromancer becomes the "boss necromancer", who then teaches other historical figures the secrets and who, in turn, teach the secrets to others, creating a pyramid-shaped chain of teaching and aprenticeship with the boss necromancer at the top.
This holds true for every tower, however there is only one tower per tablet.
The reason you may not find many towers is because all aspiring necromancers may be migrating towards the only tower with the only tablet. More civilizations means more dieties, which ups the chance of a diety who governs death and so, a tablet. Furthermore, more civs means more clusters of civilization from which more towers can spring up if far apart enough.
And then there are the zombie requirements, which are needed to build the tower.
The conditions needed for a new tower to be built, and aspiring necromancers to migrate there are complicated, but Ive found out this much from repeated world gens and looking at legends.
Oh and necromancy is not limited to humans and dwarves. Ive found amphibian men necromancers, with masterwork steel masks, masterwork robes, gloves, hoods, loincloth shoes and socks. Which is odd considering they are usually naked. The tower was filled to the brim with zombie elves, also wearing masks and robes, and similarly dressed dwarven and human necromancers.
Pretty cool if you ask me.