The Atlas V has a special way of naming the different kinds. The first number says how wide the fairing is. A 400 series Atlas V has a 4-meter wide fairing. A 500 series Atlas V has a 5-meter wide fairing. The second number says how many strap-on solid rocket boosters the rocket has. An Atlas V 501 has no strap-on boosters, but an Atlas V 421 has 2 strap-on boosters. The last number says how many engines the Centaur second stage has. All of the Atlas V rockets flown have had only 1 engine on the Centaur, but in the future other Atlas V rockets will have 2 engines on the Centaur second stage. There is also the HLV which has two of the first stage (Common Core Booster) strapped on like strap-on boosters, but it has never flown.