As with a conventional amp-and-wired-speakers, two speakers will give you better stereo imaging. But a single Era 300 is reportedly pretty good. 
 

How big is the room? What shape is it? What music do you listen to? What kit have you used before? 
 

Sonos gear is easy to reconfigure so buy 2 speakers, play around with just one and both, and return one of you’re ok with just the one. Sonos offers a generous home trial period. 

Thanks for this.  The room is a bath to a primary en suite.  Approximately 12’ x 12’ x 8’ ceilings.  I am an architect fitting out the space for an owner.  Don’t know that they have other Sonos in the house.  We are remodeling the bathroom, and the owners asked for wired speakers to their ceiling.

I love it when not all the details come at once. 
 

Do they already have an amplifier to drive the speakers? If they’ve asked for “wired speakers” why are you considering “one speaker”? Why are you considering using Sonos - is this what they’ve requested? 
 

So many more questions but I think I’ll just back out from this thread and let someone else take over…

Hope you do get it resolved. There are professional audio installers listed on Sonos website. 

“Best experience” is very personal. For example: In your profession some customers would love a rough log cabin style livingroom while others want a slick marble floor and mirrored walls scheme.

What is the purpose of the room? Quick shaves while listening to the morning news or is there a hot tub for lounging? You’ll get the best illusion of an orchestra when there are two speakers and the listener is as far from each speaker as the speakers are from each other. 

Would the listener want to hear the same or different music in the bath or primary room? In any case you’ll want to have the ability to separately adjust the Volume in the bath. Talk to the customer. What are the expectations?

I don’t know the system details, but if the other speakers are wired, I think that wired speakers for the bath would be most appropriate. If you want only a single speaker, use this approach. This speaker is wired as a stereo pair, but there is only one mounting hole. The result is not actually stereo for the listener, but the system wiring will be much easier.

Since you would bring in an air conditioning professional early in the project, consider bringing in an audio professional.

Does Sonos have a  in-ceiling speaker that is wireless?  Or are all their in-ceiling speakers wired?

(Edited to delete comment)

“Wireless” speakers need power.