In Captain America: The First Avenger, we see that the Super Soldier Serum that created Captain America was destroyed when the Nazi spy blew up the lab, and its formula lost with his dead creator, just like in most other versions across all of Marvel.
In Captain America: Civil War, however;
The Winter Soldier was sent by Hydra to ambush Howard Stark's car in order to steal multiple vials of Super Soldier Serum so that more Winter Soldiers can be created.
It appears that the formula has been rediscovered. If that is so, why didn't the Americans produce more Captain Americas? That was the original plan back in World War II after all - they wanted an army, not just one Steve Rogers.