The reference is to the seventh king (head of the beast). As the day of Christ’s return could be any time (Matthew 24:36), the future row of kings is represented by one head. Implicit is that this seventh is of the same mind as the previous kings; that is, he is in bed with the prostitute (Psalm 2:2) and so he will inevitably fall also.
10 they are also seven kings, five of whom have fallen, one is, the other has not yet come, and when he does come he must remain only a little while.