You don’t. Some TVs will show that number, and some will not.

SAMSUNG’s can be obstinate when communicating with other equipment. I suggest that you disconnect all HDMI inputs to the TV and power down TV and ARC for at least two minutes. Power up the TV and ARC before connecting only the HDMI cable between TV and ARC. Give them a couple minutes to share details. Make sure that the TV’s Anylink option is enabled. Now you can connect other HDMI cables, if any.

Maybe, the TV will now cooperate with ARC and display Volume levels. SONOS does report these levels via CEC, but TV’s can ignore the report.

My Samsung doesn’t show the volume on the Arc. The only way to know is on the Sonos app. You can’t force it on screen when the Arc is the Receiver from a Samsung.

That information is a choice made by the TV’s UI, as suggested by ​@buzz . While Sonos reports this data via CEC (HDMI-ARC), the TV makes the choice as to whether display it. It isn’t an overlay created by Sonos. 

To me the reliance on numbers when setting volume seems very strange, especially since different) content (stereo or surround) or different broadcasters will need different numbers for the same volume. This makes the volume numbers just seem objective, while they are not. The background knowledge in your home can also vary.

As you say “I just have to rely on the sound I hear to control the volume”, I would say that is just what your ears are meant to do.

Yes, THIS.