The disciples (or those closely associated with them) are traditionally credited with writing the following books of the New Testament:
* Matthew: Gospel of Matthew
* John: Gospel of John, 1 John, 2 John, 3 John, Revelation
* Peter: 1 Peter, 2 Peter
* James: James
* Jude: Jude
So, by this traditional attribution, the disciples (or those closely associated with them) contributed 14 books to the New Testament.
It's important to remember that the authorship of some of these books is debated by scholars. However, according to tradition, the number is 14.