David wrote

Sub AppendNumber()
c = 36
For i = ActiveSheet.UsedRange.Rows.Count - 14 To 3 Step -15
Range("A" & i).Value = "LASTNAME" & c & ", " & "FIRSTNAME"
c = c - 1
Next
End Sub

For my fortunately limited range (this is only a one time deal), this
works:
Sub AppendNumber()
c = 90
For i = ActiveSheet.UsedRange.Rows.Count - 14 To 3 Step -15
Range("A" & i).Value = "LASTNAME" & Chr(c) & ", " & "FIRSTNAME"
If c = 65 Then c = c - 7
c = c - 1
Next
End Sub

--
David