Avatar: The Last Airbender was comprised of 3 Seasons/“Books” – Water, Earth and Fire.

Book One, “Water” has 20 episodes and is available on DVD as the “The Complete Book One Collection”

Book Two, “Earth” has 20 episodes and is available on DVD as the “The Complete Book Two Collection”

Book Three, “Fire” has 21 episodes (counting the mini movie as the last 4) and is available on DVD as the “The Complete Book Three Collection”

All three seasons are on Netflix Watch Instantly, but some episodes duck in and out of availability.