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.

Cross-reference

Revelation 17:16 details the ten horns (kings) destroying the prostitute; 17:10 introduces the seven kings, part of the same prophetic scene.

Revelation 12:3 introduces a dragon with seven heads; the seven heads are interpreted as seven kings in 17:10, directly linking the visions.