In this context, the reference is to John, son of Zeberdee. Tradition credits him with having written five books: the Gospel of John, the three epistles that now bear his name (1 John, 2 John, 3 John) and the Book of Revelation.
However:
There are 21 letters, mostly by Paul, John and Peter.
There are three letters written by John in the New Testament - 1, 2, and 3 John.
21
21
The letters of the New Testament begin with a greeting to the brothers and sisters, followed by exhortation for them to stay strong in the faith. There is also a teaching message and a farewell.
The Gospels & Acts The Pauline Letters The General Letters Revelation/Prophecy
The 6 books of the New Testament that are not letters include: Matthew, Mark, Luke, John, Acts, and Revelation.
there are 5
There are books in the Old Testament, letters are in the New Testament.
The new testament
There are no books called "letters" (or epistles) in the Hebrew Old Testament. All books that are designated as "letters" are Greek, and found in the New Testament.
Paul aka Saul wrote the most letters in the New Testament