The "Crisis on Infinity Earths" event has introduced so many Earths to the Arrowverse. But do the numbers have any significance? Some obvious ones:
Earth-666: Lucifer's earth [666 = Devil]
Earth-66: Based on the old '66 Batman year (1966)
Earth-89: Tim Burton's Batman year (1989)
Earth-90: The old Flash show year (1990)
What about the others?
Might be a nod to the debut of Superman (1938) as mentioned in another answer.
The debut episode of future Oliver was in Legends of Tomorrow during first season (2016)
Might be a nod to DCAU (Batman Beyond premiered in 1999)
Issue #75 when Doomsday killed Superman.
The number 167 is a nod to the show's producers Alfred Gough and Miles Millar both being born in 1967. source
Must be a reference to Kingdom Come (1996)
Birds of Prey (TV series) ended on 2003
Based on comics and numbered similarly
In comics, it was wild west earth. We only see Jonah Hex (Not part of comics earth 18) who is a western hero.
Pending:
Black Lightning and Nash Wells/Pariah earth is not numbered
