You can’t add additional rear surrounds. Two Era 100 surrounds is the limit.

You can’t add more than 2 surrounds.  

And the Arc isn’t a ‘center’ speaker, it’s a center, right, and left, along with upward firing (Atmos) speakers.

Sonos doesn’t make any speakers which are ‘center’ only. 

Thanks everyone.  I guess I’ll be looking into a nakamichi surround system.  Or maybe the new Awol thunder beat.  I’m going to be purchasing a 132” projector screen and UST projector and want to have the best in picture AND sound and I think there are better options. If Sonos could add more speakers, they would have my vote.  
 

Again, thanks everyone!  

Any soundbar based system is going to have the same issue.  They all have the Left/Center/Right in the same enclosure.  That’s the very definition of a soundbar. 

If you’re this serious about your homecinema, a dedicated speakers solution will be much more suited than anything produces….

Maybe something like the Denon AVR-A1H driving these Klipsch Reference Premiere 5.0.2 Bundle, you could add more speakers as desired as well as several subs.

Way more complicated and expensive, at $10,000 plus subs, than a Sonos solution but when the sound is what matters, go for it.