Well, think about how many planets there are in the entire galaxy within the reach of existence.

3600, originally. There was 1 member for every possible sector.

Then the number grew, because they gave more Lanterns to a single sector, like 2814.2, 2814.3...

And the Corps was killed at some point, so the number is variable.

Let's say 3600 is the standard number, or the smallest number to make the Corps work properly (1 Lantern per sector).

As usual, Sisu tries to be funny with the minuscule knowledge he has about the Green Lantern Corps.

Also, with the current series, many Green Lanterns are dead or presumed dead, and possibly the Corps lost a third of their numbers. The Guardians also gave a bunch of sectors to the United Planets, and they reassigned their corresponding Lanterns to other sectors or duties.

The United Planets won't come intp play until the 30th Century.

In the present, it would generally be 2 per sector, so 7200, plus a bunch of incidentals. I doubt Greet has to patrol a sector. The same for most of the Lanterns with duties on Oa, like Salakk, Shorm, Kilowog, Morro and Ke'Haan. Add the honor guard and Alpha Lantern Corps, which would be in all, 10-50 more?