If you are happy with mono (possibly slightly improved with angled speakers) sound instead of stereo then a single Sonos speaker will work. My single Play 5 sounds pretty good as long as I am directly in front of it and less than around six feet away. Past that distance or off center it sounds pretty much mono to my ears.

Then you get to the “how loud” question, a single 100 won’t play as loud as a single 300. I think (but haven’t been able to test it) that a Five will play a bit louder than a 300. When you use two for full stereo you see the same differences.

Next you have low frequency reproduction, the 100 is not bad but the 300 and Five are better. You might find yourself wanting to add a mini-Sub to the 100s or a full Sub to the 300s. Again I haven’t been able to test this but the Five might perform a bit better lessening the need of adding a Sub. Do look for other folks ideas here as several here have been able to compare these combinations.

If you want Atmos, the paired 300s are your only option. The guide is not clear though, it seems to say two are needed but it doesn’t say one won’t do Atmos.

I don’t have access to Atmos so my optimum setup would be a pair of Fives, properly positioned for best stereo imaging. Once I had them for a bit I’d decide if I needed more low frequency support and consider a full Sub as an addition. 

Other topics I’ve seen have folks happy with the 300s and a Sub or 100s and a mini-Sub. So much depends on your room, your source and your ears.

More about your room might help. Closed or open? Ceiling height? Live or dead sound?

Same for your music tastes. Organ (my big driver) rock, classical?

Don’t rule out the Five. For “pure” music, Sonos recommend the Five over the Era 300. 

Thanks for everyone's thoughtful replies.

I decided on two Era 300's and the sound is fantastic. The 500 would have been a good choice, but not for my budget, and peace with my spouse.