Is the devil only one? Or Several?

When Satan obtained the legitimate authority in the universe, he and his angels naturally became a king together. And Satan became the king, namely the ruler of this world. When he became the ruler, his name changed from Satan to Devil. Another name for the Devil is Lucifer, which also symbolizes a king to a certain degree (Isa 14:12).

One important thing to note is that the word Devil is always written in the singular form in the Bible. Some people carelessly say, “Devils,” but there is no such word in the Bible. The Devil is not many but one. Yet, his influence is felt everywhere because his agents are operating throughout the world.

My answer refers to the book, written by pastor Ki-Dong Kim,
What are Deceving spirits, Chatper 4, “The will that Overcomes the Deceiving Spirits,” p. 64.