Complex sentence: On September 11,2002, Cutrone's agency broke the record for the most shows produced on one day during New York Fashion Week--five--mainly because she was the only one willing to produce shows on the first anniversary of the September 11 terrorist attacks.
Split into simpler sentences: On September 11,2002, the agency set the record for producing the most shows in one day (which is five) during New York Fashion Week. This is mainly because Cutrone was the only person willing to produce shows on the first anniversary of the September 11 terrorist attacks.