Revelation 17: 10: And there are seven kings: five are fallen, and one is, and the other is not yet come; and when he cometh, he must continue a short space.
The seven kings begin with Julius Caesar who became Dictator of Rome. Augustus, Tiberius, Caligula and Claudius were the successive emperors of Rome after Caesar. Nero was emperor when John recorded the Book of Revelation (“and one is”) and was succeeded by Galba who ruled for a short space of seven months.