There is only one devil (Satan), but there are many demons. The demons are those angels who sinned with Satan by following him when he revolted against God.
Some are confined (2 Peter 2:4), but many are active in the world (12: 43-45) They seek to thwart the purposes of God. (Eph 6:11-12); they promote their own system of Doctrine (9 :33); and they possess the bodies of men and of animals (4:24; Mark 5:13.
Comments: For those that may be doubting whether the spiritual world is real, and manifests in the physical world, take note of the above scripture…Frankly my dear… good and bad angels are at warfare all around us, this earth is the battleground, and we are the players and the prize… Satan is the prince of the power of the air. From my personal Bible study.