Question about Sanus WSWM22 Wall mount for Sonos One, Play:1 and Play:3, Pair

That is correct, you are buying a pair. And with the product with the end number 21, a single bracket. I also had this product (22, pair) and was very satisfied with it, the installation is very simple (use: rear speakers for home cinema).

By the way, it's best to wire everything with LAN and deactivate WLAN in the Sonos app. The connection is much more stable and you reduce the electromagnetic pollution in your home.