This verse refers to disciples (not the 12) walking away from Jesus's teachings. The reference numbers for this Bible verse from the book of John are interesting considering the subject matter.

Note: Nero was the emperor of Rome when John wrote the book of Revelation. The Hebrew alphabet at the time of Jesus, also had corresponding numbers. The numbers that corresponded to Nero's name in Hebrew then are 666.

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

Some people (exp. Bible Project) view the connection of 666 to emperor Nero's name to represent, not specifically Nero himself, but a broader meaning of empires and world leaders who oppose Jesus Christ as the world's rightful King of kings.