Who wrote the most books of the Bible?

Modern scholars credit the anonymous author now known as the Deuteronomist with writing a total of seven books: Deuteronomy, Joshua, Judges, 1 Samuel, 2 Samuel, 1 Kings and 2 Kings.

The apostle Paul is traditionally credited with writing thirteen books, but there is a near-consensus among scholars that he only wrote seven epistles: 1 Thessalonians, Galatians, 1 & 2 Corinthians, Philippians, Philemon and Romans.
The Apostle Paul wrote more books of the standard Christian Bible than anyone else--thirteen. He is the author of Romans, 1 and 2 Corinthians, Galatians, Ephesians, Phiippans, Colossians, 1 and 2 Thessalonians, 1 and 2 Timothy, Titus, and Philemon.
Paul may also have written Hebrews, but no one knows that for sure.

Apostle Paul's work can be found in the New Testament.

Moses wrote the most books in the Old Testament. He is credited with writing six books: Genesis, Exodus, Leviticus, Numbers, Deuteronomy, and Job.