Mark 5:9

What is thy name? Why Christ asked the name of the spirit possessing the man, or rather the spokesman for the legion of spirits, is not clear. It has been suggested that this was for the benefit of the disciples, in order that they might appreciate more fully the magnitude of the miracle, and better realize the nature and power of the forces against which they must contend.

Legion. A Roman army division, which, at full strength, consisted of approximately 6,000 footmen and 700 horsemen, or a total of about 6,700. Commonly, however, as with modern armies, the legion was not maintained at full strength. Though the demon’s use of the name Legion may be taken literally, there is no way of determining the precise number. The expression is best understood in the general sense that there were many demons (see Luke 8:30).