You would want to pick up a speaker impedance matching device, that will do the job.

Just keep in mind the more speakers the less loudly each will play.

If you use the Sonance speakers sold by Sonos, the Amp is stable with 3 pairs.  Anything else over 2 pairs at 8 Ohms and the Amp will shut down.  

@jgatie sorry to highjack, you say 3 pairs, is that in parallel?

Would I be OK with 2 pairs, but  8ohm (Sonance) and some 6ohm bookshelf?

https://en.community.sonos.com/components-228996/amp-with-2-pairs-of-speakers-of-different-nominal-impedance-6846219

All 3 pairs must be the Sonance speakers that are made for Sonos, wired in parallel.  Otherwise you are limited to 2 pairs of 8 Ohms impedance. 

@jgatie,

I was looking at the Sonance set that comes with the Sonos Amp, and adding two more sonance speakers to this in parallel. 

The 4 smaller speakers around the patio and the two larger ones located on the deck.

Sonance - MAG 4.1 Outdoor Streaming Music System

Sonance - Mariner54 5-1/4" 2-Way Outdoor Speakers (Pair) - White

You should be able to run the 4.1 setup off a single Amp, as it presents a 8 ohm load on the amp, and I don’t know why it would be sold as a package if it didn’t work.  You aren’t going to be add more speakers to that setup, unless you use impedance matching speaker switch.

Also, I imagine this speaker package is going to be the rare case where it should be wired in series rather than parallel...hopefully instructions provided.

Yes the package will work as the Sonos amp says it will power 2 pairs (4 speakers) wires in parallel.  
 

my question was if I add another pair or speakers in parallel will the amp be stable?

No.