How many books are missing from the Bible?
None. Mt 5:18 For verily I say unto you, Till heaven and earth pass, one jot or one tittle shall in no wise pass from the law, till all be fulfilled.
Jesus says that every dot and stroke is important enough for him to protect. Theologians have an imagined document called “q” which supposedly predates Matthew, Mark, and Luke, and which those authors borrowed from. The problem is that it doesn’t exist. Even if it did at one time, it no longer does, and so it was not under the protection of God.
God gave us a way to determine which existing documents are scripture. It is called ‘sensus plenior’. It is like security paper. (1) He wrote a a second story hidden in the first story. If you distort the literal story, it would distort the hidden story as well. Books which are not scripture do not have this second layer.