The number of songwriters needed for a hit song is as many as it takes. That could be one or seven depending on the circumstances. Now let’s ask how many people does it take to make a hit movie? Just saw Iron Man Three. I like to sit through the credits. There must have been literally a thousand or more people to make that movie. But that is what it took to get it out. The same is with a song. The seventh songwriter, no matter what their contribution was needed to get the song out.