That argument is a slippery slope - how many is the right number?
The strongest argument is for exactly one. One Green Lantern was how it was created; for 10 years it was just Alan Scott. The first 25 years of Hal Jordan were the same - and the modern GL mythos was almost entirely defined halfway through that run. Adding more Earth GLs in the 1980s didn't really expand the mythos - it just became more of a team instead of a solo character. And the cast has been a rubber band (shrinking, expanding) ever since.
So how many do we "need"? One.
But some good stories got told with every expansion. So where do you draw the line? Most people would do it with their favorite character - but it doesn't work that way. Creators are free to try new ideas, new characters. And the market responds. In 2010 it was, "Simon Baz is kinda interesting." In 2020 it might be, "whatever happened to Simon Baz?"