Four 8.5 or 9 x 20 wagons will hold 900 bales thats the same number of wagons you have correct. Go with 6 you could get 1200 at a time. If you park them in the field when you start baling someone could have them loaded when you stop baling then go to barn. Heres a photo of the type of wagon we use all homemade out of native lumber with some steel.