https://www.quora.com/Is-it-true-that-the-number-666-is-related-to-Roman-Emperor-Nero
in the first century AD, the Emperor Nero *specifically* was seen as a horrible person and persecutor of Christians. Christians of the first century had such strong negative feelings towards Nero that some even feared that he would come back from the dead to terrorize them more. To Christians in the first century, the imagery in Revelations would have been obvious. The “Whore of Babylon” who sat on seven mountains (Revelation 17:9 “The seven heads are seven mountains, on which the woman sitteth.”) would have obviously represented the city of Rome itself, which famously sat on seven hills.