What, according to the Book of Revelation, is the "number of the Beast"?

"666" is given as the number of the beast (of multiple heads and horns) described in the Book of Revelation (and generally associated with the devil. In Greek, individual letters also had numerical values. The value of the letters that spelled out the Emperor Nero's name in Greek add up to 666. The first serious persecution of ancient Christians by the Roman Empire occurred while Nero ruled. In antiquity, "7" was seen as a "perfect" number and "6" as an "imperfect" number. 666 was triply imperfect.

https://en.wikipedia.org/wiki/Number_of_the_beast

From Revelation 13:16-18

16 It also forced all people, great and small, rich and poor, free and slave, to receive a mark on their right hands or on their foreheads, 17 so that they could not buy or sell unless they had the mark, which is the name of the beast or the number of its name.

18 This calls for wisdom. Let the person who has insight calculate the number of the beast, for it is the number of a man. That number is 666.

https://www.biblegateway.com/passage/?search=Revelation+13&version=NIV