Revelation 17
8 The beast that thou sawest was, and is not; and shall ascend out of the bottomless pit, and go into perdition: and they that dwell on the earth shall wonder, whose names were not written in the book of life from the foundation of the world, when they behold the beast that was, and is not, and yet is.
9 And here is the mind which hath wisdom. The seven heads are seven mountains, on which the woman sitteth. Rome
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 Caesars, Augustus to Galba
11 And the beast that was, and is not, even he is the eighth, and is of the seven, and goeth into perdition.
so there is your answer, the "8th king is actually one of the 7
The bible will answer most questions if you just read the CONTEXT.