We have a Youth Fair coming in March.They have a livestock exhibit/show and I've been told that there are Leghorns shown every year there. I plan on checking them out.In my area with leghorns of course whites are the most entered with light browns being a close second. There are very very few of the other varieties shown around here.
As for how common compared to all the other breeds. They're somewhere in the middle I suppose. Lots of more popular breeds but lots of breeds aren't even represented at all.
Oh how I miss the shows back in the 70s and 80s.