You can add another case selection e.g.
Select Case ContentControl.Title
Case "checkbox1" 'The title of your CC.
For lngIndex = 1 To 299
On Error Resume Next 'ensures missing numbers do not force an error
Select Case lngIndex
Case 1 To 12, 209 To 214, 249 To 254, 297 To 299 'The groups of numbers separated by commas
If ContentControl.Checked Then
ActiveDocument.SelectContentControlsByTitle("textbox" & lngIndex).Item(1).Range.Text = "N/A"
Else
ActiveDocument.SelectContentControlsByTitle("textbox" & lngIndex).Item(1).Range.Text = vbNullString
End If
End Select
Next lngIndex
Case Else
End Select