That's exactly what I asked myself when I saw the double page spread in Blackest Night #1. After consulting the Green Lantern Secret Files & Origins issue that came out around the time of the Sinestro Corps, I've managed to identify 45 different Green Lantern characters in this image. Click below for more:
Here we have the image, numbered with the names of the Lanterns at the bottom. If you have trouble clikcing on the image to enlarge it, please try the link below the image and make sure to click the Full Size button once you are in PhotoBucket.
If I've missed any or mis-named anyone, please let me know :-)
DIRECT LINK TO LARGER IMAGE