It’s a bizarre UI/UX implementation by Sonos to identify how many targetable rooms are being controlled in the ‘group’.

If you group two rooms together, the speaker icon will show a 2, three rooms grouped will show a 3 etc which makes sense.

There is zero need for a ‘1’ to be shown when the room is not grouped with any other room.

Or 

Reading the links above just provides further posts with users questioning the need for the “1”.

Can add the following thread to the reading list too.