What is hotkey in Linux Mint for "Show All Windows"?

For Linux Mint 18 Sarah ,<ctrl+alt+down-key> worked to display all the open windows in the current desktop. To view all the windows in all the desktops, use <ctrl+ alr+ up-key>.

Should work for Ubuntu too.

I think its name is "Toggle Scale" on the keyboard shortcuts settings.

Yes, its toggle scale under general in keyboard shortcuts.

Default is CTRL (ALT/shift) down, but not to sure.