According to the Historia Brittonum, Arthur fought twelve battles.
The claim appears in chapter 56 of the work, an Arthurian section that describes Arthur as a dux bellorum, or military leader, rather than explicitly calling him a king. The text lists twelve victories, but many of the place-names are obscure and cannot be identified with certainty today.
The most famous battle in the list is Mount Badon, although the Historia Brittonum does not provide dates for Arthur’s campaigns. Later writers and historians tried to connect the battles with real conflicts in post-Roman Britain, but the evidence is too uncertain to establish a reliable itinerary or prove that a historical Arthur led them.
The Historia Brittonum was compiled around the ninth century, centuries after the period in which Arthur is traditionally supposed to have lived. It is consequently an important source for the growth of Arthurian legend, but not a straightforward contemporary record. The number twelve is the text’s own total; it should not be confused with other medieval versions that add, alter, or reinterpret Arthur’s campaigns.