Two can be set up as surround speakers. Up to 31 can be ‘grouped’ for synchronous music, although they’ll be delayed by at least 75ms for any TV data that the Arc is playing.

None can be ‘paired’ (but that’s just technically speaking, as that term is usually used when stereo-pairing two very similar identical Sonos products), but you can ‘bond’ two One-SL’s as surround speakers and you can also ‘group’ up-to 31 One-SL’s to the Sonos Arc, as there is a limit of 32 Sonos devices per Sonos System/Household.
 

Note: If you ‘group’ speakers to the Arc, all will play in perfect sync for music audio, but when playing TV audio the ‘grouped’ devices will have around 75ms latency and if used in the same physical room you ‘may’ notice an echo effect in that room, also the grouped rooms maybe out of lip-sync with the on-screen video. That’s not the case however with the two ‘bonded’ surrounds I mentioned earlier, as ‘bonded’ speakers will play in sync for all the available audio types.

edit: Ah, Bruce beat me to it, he clearly types much faster than me.