User blog comment:Kaido King of the Beasts/Chapter 908: Imminent, Imbalanced, Improbable Whims/@comment-4783583-20180615102625/@comment-25501246-20180616000034

"Fiftly the world government is made up of over 170 individual states, fifty of which are kingdoms, the other ergo are not kingdoms. Also there were exactly 50 rulers in the room, I counted."

The chapter confirms that the 50 Kings are selected.