I love maps in my fantasy novels. There was a period in my teen years when I wouldn't check out a book unless it had a map. A detailed, colorful one, preferably. Looking at so many maps over the years, I've picked up on a few trends. The most recent one I've noticed: seven kingdoms, seven realms, seven countries.
Often the number is included in the official name of the world. Sometimes it's just a descriptor. Seven has always been popular with fantasy writers, I think (J. K. Rowling has said the number has always held a special magic for her, and she's not alone), so I suppose it's not surprising to see. But a reader can't be blamed when the worlds all blend together in her head as a result.
Links lead to places where you can see the official maps. I feel certain there are some I missed - let me know in the comments.
The Seven Kingdoms of Westeros
from the Song of Ice and Fire series by George R. R. Martin
I suspect some of the popularity of the seven kingdoms/realms/countries can probably be traced to Martin. Despite the much-used phrase "the seven kingdoms," even some fans are confused as to what the seven kingdoms really are - the seven countries of the region before they were conquered and united by Aegon, which happens before the book series begins.
The Seven Kingdoms
from the Seven Kingdoms trilogy by Kristin Cashore
It's not only the name of the world, but the name of the book series as well.
The Seven Realms
from the Seven Realms series by Cinda Williams Chima
Same for Chima's series, which I'm sure I'll get around to reading some day.
The Seven Kingdoms
from Prophecy by Ellen Oh
By the time I got to this book I was thoroughly confused (though Oh's series is refreshingly based on a historical culture of the East rather than the West).
The Seven Kingdoms of Annar
from the Pellinor series by Alison Croggon
I've had the first book in this series sitting on my shelf for a while, but haven't dug in.
The Lumatere Chronicles barely escapes this list with eight countries in its world of Skuldenore (though it can be argued that with Lumatere in such an ambiguous place for most of the book, it essentially only has seven).