The number of Citi ThankYou points you need for a free flight depends on the dollar price of that flight, as your points are worth 1 cent each when redeemed for flights through the ThankYou center. This means that you'd need 40,000 ThankYou points to purchase a $400 flight through this portal. Alternatively, you can transfer your ThankYou points to one of Citi's airline transfer partners. The number of Citi ThankYou points you need in that case will depend on the airline partner and how many miles you need with that partner to purchase the flight you want.