Christopher Tolkien’s The History of Middle-earth contains 12 volumes.
Published between 1983 and 1996, the series presents and discusses drafts, revisions, notes, and other writings created by J.R.R. Tolkien. Christopher Tolkien edited the material after his father’s death, arranging it to show how the mythology of Middle-earth developed over many decades.
The books cover subjects ranging from the earliest versions of the mythology to the histories of Númenor, Beleriand, and the later Third Age. They are not conventional novels and often include multiple versions of the same story, editorial commentary, and unfinished passages.
The 12-volume count can be confused with other Tolkien posthumous publications. Unfinished Tales is a separate single volume, while The Children of Húrin is a standalone edited narrative. The History of Middle-earth is specifically the large documentary series tracing the evolution of Tolkien’s legendarium.