Col. Trevor Dupuy's classic study shows that for the American Civil War, the average ratio of wounded to killed was 4.55 to 1 and the ratio of surviving wounded to battle deaths was 2.38 to 1. Of course some of those surviving wounded would be permanently disabled and discharged. I don't know of any study that specifically tried to determine the percentage of the surviving wounded that returned to the ranks. Of course there are many documented cases of individuals being wounded multiple times and returning to fight again each time.