Correct answer: FOUR
In addition to Paul, other writers of general epistles in the New Testament were Peter and John (apostles) and James and Jude (brothers of Jesus and leaders in the church).'
The reason for excluding Hebrews from your list is that we don't know for certain the authorship of that epistle. Among the church fathers, Augustine thought it also was written by Paul, but Tertullian attributed it to Barnabas (a coworker with Paul). Martin Luther suggested it may have been Apollos, another of Paul's coworkers. Early 20th century theologian Adolph von Harnack proposed that Hebrews was written by Priscilla, a female coworker of Paul's -- and that her being a woman was the reason that the author's name was not maintained by the church later.
