I, the shepherd, have sinned: Earlier, David confessed his sin only about his situation ("take away the iniquity of Your servant," 2 Samuel 24:10). Now, he prayed for those he thought were innocent (but see 2 Samuel 24:1) and offered to die for them.
these sheep: David was the shepherd (a leader who guided, protected, and cared for the people) of Israel (see study note on 2 Samuel 7:8).