Just generated a pretty standard small world today. After 250 years has 16 necromancy Towers!
(https://i.ibb.co/p0FvL91/tower-Markup.png)
(https://i.ibb.co/HFYvLR8/tower.png)
Download the world here: https://dffd.bay12games.com/file.php?id=15124 (https://dffd.bay12games.com/file.php?id=15124)
It doesn't seem to be that uncommon. I just tried generating a small world and after 250 years it has a grand total of 47 towers, 19 of which are occupied by necromancers (purple) while the other 28 have been conquered by other civs.
(https://i.ibb.co/chNsXfL/region3-00250-01-01-world-map.png)