The Bible clearly portrays them as personal beings. Demons are not omnipresent nor do they have unlimited power. Since demons are fallen angels, like angels they are immaterial and not usually subject to human visibility or our other senses. Demons are not all-knowing. They are pretty good at predicting human behavior, however, because they are experts on human nature.
Demons are numerous.
This means that there are many, many demons. Nevertheless the number of demons is far less than holy angels who care for God's people. No, a Christian is owned by God. We recognize the influence, actions, or oppression of demons in the lives of many believers. Jesus saw the need for deliverance from demonization.