It’s amazing the information Sonos provides on their website. This from the FAQ’s for the Ultra:

Which Sonos speakers work as surrounds with Arc Ultra?

A matching pair of the following speakers will work as surrounds: Era 300, Era 100, Sonos One (including SL), Sonos Five, and SYMFONISK (Gen 2) speakers. You can also use Amp to power in-wall, in-ceiling, or other passive speakers as surrounds.

Note the Pair, you can't use a single anything, must be two matched speakers.

Two matched, compatible speakers.