It is explained by Sandaime Hokage that, though there is one winner of the tournament, it doesn't guarantee that he will become a chunin.
He also explained that there can be more than one chunin from an exam and there can be none too.
If there was only one chunin per exam, it would have been impossible for all of Naruto's friends to become chunin after his 3 years of training with Jiraiya.
