Does this answer it?
*** w09 6/15 p. 22 par. 10 The Faithful Steward and Its Governing Body ***
10 However, were all spirit-anointed Christians involved in overseeing the preaching work and in preparing spiritual food? No. The apostle Paul tells us: “Not all are apostles, are they? Not all are prophets, are they? Not all are teachers, are they? Not all perform powerful works, do they?” (1 Cor. 12:29) Although all spirit-begotten Christians engaged in the preaching work, only a very limited number—just eight different men—were used to write the 27 books of the Christian Greek Scriptures.