Simultaneous Back up to iCloud?

Yes. If you turn on iCloud Drive on your Mac and save the file to the iCloud Drive folder it will automatically sync to iCloud. After you turn on iCloud Drive on your Mac you can access the iCloud Drive folder via the Finder "Go" menu. You can also add it to the Finder sidebar via Finder Preferences.

See: iCloud Drive FAQ - Apple Support

To make sure iCloud Drive is enabled for Numbers got to System Preferences > iCloud, make sure iCloud Drive is checked, click the 'Options...' button next to it, and make sure Numbers.app is checked.

If you're using Numbers on your iPad/iPhone the process is similar there.

iCloud Drive stores a copy on your hard drive for when you don't have an internet connection but at the same time will keep a copy "up in the cloud" on iCloud Drive.

SG

Yes, actually saving it in multiple place. A local copy on your machine. A copy in the cloud. And a copy to other devices if you've configured them to do so. When all is working well (and for me it's been very solid for several years) a seamless experience, particularly with Numbers.

You can add iCloud Drive to the sidebar in Finder by going to Finder > Preferences > Sidebar.

SG