For the Atlas V it is just one RD-180 engine per launch "The Atlas V first stage...is powered by one Energomash RD180 main engine..." [0]. There are additional

For the Atlas V it is just one RD-180 engine per launch "The Atlas V first stage...is powered by one Energomash RD180 main engine..." [0]. There are additional solid rocket boosters (supplied by Northrup) on the first stage. And Aerojet Rocketdyne provides the engines for the upper stage. Something about the math still doesn't quite add up though. I wonder if the 24 engines are still the ones sitting on the shelf, while others have already been integrated in to the first stage of the rockets. [0] https://en.wikipedia.org/wiki/Atlas_V#Vehicle_description https://en.wikipedia.org/wiki/Atlas_V#Vehicle_description Edit: It's also a bit odd because one RD-180 engine actually has two nozzles.