Given only to those who finish the challenging course, the t-shirt is a big reason why so many of the 55,000 run the race. what is the best way to paraphrase sentence (6)?

The t-shirt that they give only to people who finish is a big reason that so many people take up the challenging race. 

This paraphrase presents the same information as in the sentence, but since it should be a bit more of a summary, it takes out the specific detail of how many people actually run the race, which can just be paraphrased into 'so many'.