John Gill Commentary Luke 8:30

"And Jesus asked him, What is thy name? And he said, Legion; for many demons were entered into him." — Luke 8:30 (ASV)

And Jesus asked him, saying, what is your name ?
&c.] This question was put, not out of ignorance in Christ, but for the sake of those that were with him; and partly, that the miserable condition of this man might be the more known; and partly, that his own power might be the more manifest in the dispossession:

and he said, legion, because many devils were entered into him ;
(See Gill on Mark 5:9).