It is explained by Sandaime Hokage that, even if there is in final one winner in the tournament, it doesn't mean that he will become chunin.

He also explained that there can have more than one chunin in one exam but there can also have none.

If there was only one per exam, it would have been impossible for all the friends of Naruto to become chunin when he was training between 2 and 3 years with Jiraiya.

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.