There are seven in the first series, however the second series is still being published.
In the first series there are Seven books, however there are 2 or more books in the second series that I know of. The first two are Phoenix and Lion
As of now, five.
As of 2010, there are five books in the Fablehaven series.
There are five books that compose The Chronicles of Vladimir Tod Series.
There are five books in the Gooney Bird Greene series by Lois Lowry.
The Lockwood & Co. series by Jonathan Stroud consists of a total of five books.
So far, there are five books in Kazu Kibuishi's Amulet series.
There are five books in the Avalon series by Marion Zimmer Bradley.
There are five books in "The Sisterhood of the Traveling Pants" series by Ann Brashares.
There are five books in the Children of the Promise series by Dean Hughes.
There are a lot of books by Enid Blyton, such as The Famous Five series, The Secret Seven series, The Five Find-Outers series, and many more. Like the Noddy series, some of which are reviewed here noddybooks.blogspot.com
There are a total of five books in the Tairen Soul Series by C.L. Wilson.