Each of the ten kings in his own division and in his own
city had the absolute control of the citizens, and, in most cases,
of the laws, punishing and slaying whomsoever he would. Now the
order of precedence among them and their mutual relations were
regulated by the commands of Poseidon which the law had handed down.
These were inscribed by the first kings on a pillar of orichalcum,
which was situated in the middle of the island, at the temple of
Poseidon, whither the kings were gathered together every fifth and
every sixth year alternately, thus giving equal honour to the odd
and to the even number.
6 and 5 are also references to Atlantis due to Poseidon worship.
https://www.sacred-texts.com/cla/plato/critias.htm