There are two major manuscripts of Revelation floating around. Some say the number of the beast is 666 while others say it’s 616. The author John tells us that someone with wisdom should “calculate” this number and that the number equals a man (Rev 13:18).
While there are many scholarly theories as to what this number means, the major one among academics has to do with gematria. Gematria was a form of math in which Hebrew letters equaled different numbers. This being said, there are two different ways to spell “Nero” and based on those spellings, his name will either equal 666 or 616. It’s reasonable to think that the scribes who copied Revelation changed the number based on the way they thought any given community would spell Nero’s name.
This number was not a secret for the future, it was a number John expected those reading his letter to understand. In order to not draw attention and extra persecution from the government, he masked Nero’s name in a way that he expected the church to understand. And since the book of Revelation goes deep into the topics of government and countries and the false gods and leaders that rule them, it’s no surprise that he would reference Nero.
There’s no need to be freaked out by the number 666 when it appears in random places today. Satan isn’t so stupid as to simply announce his presence to the world. The context of this number passed long ago, though the meaning lives on in the heart of that which is anti-Christ.