That's a difficult question... the Hebrew Tanach usually thought to be the most valid rendition of the Canon contains 24. But one must understand the Hebrew Scriptures are divided only into Torah (The Law), Nevi'im (The Prophets, including the so called Minor Prophets), and the Ketuvi'im (The Writings, Psalms, Proverbs, etc.). The Protestant Old Testament contains all of these, but are broken down into individual Books, some of which are further divided into 1st and 2nd (Samuel, Kings and Chronicles are examples). At any rate this Old Testament contains 39 Books.
The Roman Catholic Old Testament contains 46 Books since, not only do they add the 1st and 2nd as referenced, they also add additional Books that are considered non-canonical, such as 1st and 2nd Edras, 1 Macabees and 2 Macabees. The Eastern Orthodox Church further compounds the situation by adding an additional Psalm (151) as well as two more Macabees texts...