You can only add two surround speakers to a Sonos home theater setup. I would use the Era 300s as surrounds with the Arc and use the One SLs in another room.

 Is the listening position in the middle of the room or is it against the back wall?  

Seating is in the middle of the room. Man that sucks that I can’t use all of my speakers in the same room. Very disappointing.

So what’s the maximum amount of speakers I can use in my theater room? Just one Arc, a pair of Era 300s, and 2 subs? 

Yes. Having said that, a good sound system does not mean filling a room with speakers. The Arc, Sub and Era's will do nicely.

While you can’t add any more speakers to the surround, you could Group the ONE SL’s while playing music.