The Protestant Bible contains 66 books. This total is conventionally divided into 39 books in the Old Testament and 27 books in the New Testament.
The number reflects the Protestant biblical canon, which generally follows the Hebrew Bible's core Old Testament collection while counting several books separately. The New Testament has the same 27-book structure across the major Christian traditions, including the four Gospels, Acts, 21 epistles, and Revelation.
A common source of confusion is that Christian traditions use different canons. The Roman Catholic Bible usually contains 73 books, including seven deuterocanonical books, while some Eastern Orthodox traditions contain additional books. Early Protestant editions, including the original King James Bible, sometimes printed the Apocrypha in a separate section, but those books were not treated as canonical. Thus, 66 is the standard answer for the modern Protestant Bible.