The name of it in Japanese, Haou, well the "Ha" part of it is taken from Chinese, where it's called "Ba" and this word was used to describe the Five Hedgemons during the Spring and Autumn Period of the Later Zhou Dynasty. The "Ba" were the premier noble of the time, basically ruling over the, by that time, figurehead King, and making sure the Dynasty went smoothly. But it wasn't five men simultaneously, it was in succession.
If it was simply just "Oushoku" or "King's Haki", I think it would be a bit simpler. But to be a Supreme King, well, that feels like it really should only be one or two people alone. A lord of kings, essentially.