No. 

Hey there — I tried opening the sonos surround discussion but hit an internal error. If you want, I can help you draft a reply for when it’s fixed; just tell me what issue they’re raising (audio sync / speaker placement / connectivity etc.).

The Arc Ultra provides the 3 front channels. A single pair of Eras as surrounds gives you the 5.x, and of course the Sub makes up the .1 creating a 5.1 system. There’s no option for more speakers. 

If you go beyond the old 5.1 sound and look at Atmos and the Ultra gives you 9.1.4 and adding a pair of 300s gives six more channels each in the rear.