Hi ​@Jtiger, welcome to the Sonos Community!

It’ll still be 9.1.4, but the Era 300s and the Sub will handle parts of those channels, where; the 9 will be split between the Arc and Era 300s, the 1 will be the Sub, and the 4 will be again split between the Arc and Era 300.

I hope this information helps!

I had this same question. Thanks for clarifying. 

-PH

Note that since Atmos is a positional codec, the concept of “# of channels” goes away.  The codec calls for a sound to be positioned in a room, not played through a discrete channel, and what channel(s) are used to play that sound in that particular position varies according to the room dynamics.