For thou hast magnified thy word above
all thy name. - Psalms 138:2 KJV
Catholicism
Why 66?
By YBible
Catholics/Orthodox add the Apocrypha to the Old Testament making 73/~76 total books in their Bible.
How do we know the Apocrypha isn't inspired?
Jesus affirmed the Hebrew canon in Luke 24:44 (the law, prophets and psalms. Jews still divide their Old Testament into those three parts) & Luke 11:51/Matthew 23:35 (martyrs from Abel in Genesis to Zechariah in Chronicles: the last book in the Jewish order). No Apocryphal books or events are included.
New Testament writers directly quote the Old Testament roughly 290 times, & more broadly 800-900 times. Not once are the Apocryphal books referenced.
Isaiah foretold the completed canon. Isaiah begins with a reference to Genesis 1:1; chapter 39 ends with judgment (Malachi 39th book of the Bible); Isaiah 40 opens with "The voice of him that crieth in the wilderness" (John the Baptist 40th book - Matthew); & Isaiah 66 ends with a new heavens & new earth (Revelation 66th book); 66 total chapters, not a coincidence.