Many Civil War units of both sides sustained horrendous casualty rates - over 50% - and continued functioning - for that battle. For example, the "Iron Brigade" at the first day's fighting at Gettysburg. However in the Civil War, casualties in regiment and brigade sized units were not generally replaced. Instead, new recruits went into newly formed brigades.

So units would shrink in size as they took casualties over the months and years. I don't think the "Iron Brigade" was an effective fighting unit after Gettysburg, because it was so shrunken in size.