First, a warning/some advice. Human bodies are great at blocking wifi signals. Get the Ethernet adaptor and hardwire them. 
 

I’d say at least 2 in each room. (At least it’s easy to add more if you need them.) Personally I’d be more inclined to use Era 300’s as I think they’ll spread the sound better. Use the new Zones feature when you set them up. 

Thanks for the response!

That’s a really great piece of information that I wasn’t aware of. The thing is, it may be a challenge to go with a wired solution. Would setting up a Wi-Fi mesh network provide a solid wireless solution?

Regarding the Era 300, do you think I would need a pair for each room, or might one be sufficient?

I suspect two, but it’s easy to just add extra so start with one if you prefer. Don’t stereo-pair them if you do use two (or more). I’d still recommend a hardwire setup. Plastic conduit is quite good for neatly hiding cables. But again, try it and see. I assume they’ll be mounted fairly high to keep them out of the way?

Probably yes, but I’m not quite sure yet. Do you think this could make a difference in its/their performance?