Rehearsing my presentation on spreadsheets for the next Beaminster meeting, I found that on opening Numbers on my MacBook Pro all that will open when I select New from the File Menu is a plain spreadsheet. There seems no way to get to the Templates. This doesn't happen on my wife's MacBook Air - both machines are running Sierra. Nor on my iMac, running El Capitan.
I can offer half an answer: There is a preference to choose whether Numbers opens new documents with the Template Chooser or with a particular template. That's at Preferences > General > For New Documents (right at the top).
If you have the preference set to open a particular template for new documents, you can still get the chooser instead using Option-New (or Option-Command-N).
As to why the preferences are set differently on the various Macs, I couldn't say (and shouldn't speculate...).
That was't half an answer! Problem solved - thanks Tony! How that preference came to be set I have no idea - I am certain that I didn't make the choice of a blank spreadsheet instead of Template Chooser.
Michael Corgan 