A legion: Definition of legion

(Entry 1 of 2) 1 : the principal unit of the Roman army comprising 3000 to 6000 foot soldiers with cavalry Caesar and his legions defeated the Gauls. 2 : a large military force especially : army sense 1a the French Foreign Legion 3 : a very large number : multitude won him … a legion of devoted followers— Irving Kolodin 4 : a national association of ex-servicemen the American Legion https://www.merriam-webster.com/dictionary/legion

In Mark: Jesus addresses the men as if they are possessed by a single spirit, but the demon's answer, giving his name as "Legion," shows that he led a company of demons. Mark records that the swine that the demons entered after their exorcism numbered about two thousand. The demon was referring to the number of demons that possessed the men. This does not mean that the word Legion is demonic. It is simply a large number.