Option to Exit Macro Via Message Box

FYI, all....

Since you only really need to "do" something specific if one answer is given, you can slim that down to one command:

Sub CountWorkbooks()

   If MsgBox("More that one workbook open." & vbLf & "Workbooks Open = " & Windows.Count _
       & vbLf & vbLf & "Do you wish to abort?", vbYesNo + vbCritical) = vbYes Then Exit Sub

End Sub