On macOS, if there are multiple notifications showing, how to dismiss them all at once? Sometimes I don't touch a Macbook 12 inch because I mainly use the Macbook Pro, and after a day or two, there can be many (8 or 9) notifications at the right hand side. It seems that I have to click many times to dismiss them one by one. 

How can they be dismissed all at once?
You should be able to show the notification center (top-right icon, looks like a list), which shows you your historic notifications grouped by day.  There's a little x at the top right of each group, which dismisses the entire day's notifications.

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