You need two Sonos Fives to create a surround setup to connect to the Arc. A single Sonos Five is not enough. Or two Sonos Ones would work, too. Surround speakers must be a matched pair, and cannot be Roam or Move speakers.

Hi @Alo7 

Welcome to the Sonos Community!

In the Sonos app, select Settings » System »  Room containing Arc] » Setup Surrounds/Sub

More details are on our Add surround speakers to a Sonos home theater help page.

As @Airgetlam mentioned, you’ll need two surround speakers that match each other (Sonos One SLs would be recommended, but Fives will work too).