Sorry for taking so long. I tried to look for older posts on rays . . . don't understand how to use the "search". While I keep looking, in the mean time; . . . imagine a line from the mouth to the tail. In that picture you will see/meet the center ray. This ray branches into 8 rays. I know it's not that clear, but enough for you to see/count the end rays. If you look at 2 and 3 rays below that center ray, you will see that they only branch into 4 rays.
When viewing a live fish (or unclear pictures), you can't really count the rays. They will never keep still and pose for you, LOL. So you have to memorize the shape of 2 - 4 - 8 - more rays to "guess" the ray count.