Wendy sells vending machines. She can sell 9 machines if she charges $1,000 per machine; 8 if she sells them at the price of $1,100 per machine; 7 at the price of $1,200 per machine, and 6 at the price of $1300 per machine. Each vending machine costs $500 to produce and bring to the market. How many vending machines should Wendy sell if she wants to maximize her profit?