A pair of One SL speakers is usually a good choice for the rear channels.
Mounting depends on where you are putting them on the wall and will you have power available and where.
@Baxter46387 What model TV do you own? Is it equipped with HDMI eARC? Are you using any external media devices?
If mounting the speakers doesn’t work for your setup, consider using speaker stands for the One SLs.
FYI… if you require more bass, you can add two Subs to the Arc as long as one of the Subs is a Gen 3. You can also use Sonos Fives as rear surrounds but they are usually considered overkill for most setups.