Luke 8:30

And Jesus asked him, saying, what is thy name ?.... 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 .