Notification center documentation for applescript
Script Editor app has a searchable library. Open that under the window menu and then select the Standard Additions bundle. If you search for the term notification, you'll see it's defined under User Interaction:

display notification [text] : the body text of the notification
  [with title text] : the title of the notification (default is the name of the calling application).
  [subtitle text] : the subtitle of the notification
  [sound name text] : the name of the sound to play


But Notifications Center doesn't have any windows. Are you trying to count all windows for all applications?

To be clear - here's the notification part setting aside what a window count might be:

on run
    tell application "System Events"
        tell process "Notification Center"
            set theseWindows to "a b c"
            display notification (count of theseWindows)
        end tell
    end tell
end run