Rocky Mountain Tire Center sells 20,000 go-cart tires per year. Ordering cost for each order is $40, and holding cost is 20% of purchase price of tires per year. Purchase price is $20 per tire if fewer than 500 tires are ordered, $18 per tire if 500 or more - but fewer than 1,000 - tires are ordered, and $17 per tire if 1,000 or more tires are ordered. How many tires must Rocky Mountain order each time it places the order?