my A/C and heating unit is in my attic and my air return in high up on my 20' wall. How many filters do I change. Also, when I moved into my unit the old air filter was 16x20x1 but someone wrote 20x20x1 on the cover. The 16x20 fits perfectly. What size do I need for the Ducane EPB?

You need a filter for every return grill you have or just one at the blower section. If the 16x20 fits perfecly than that's the one to use.