Electricity

Sonos speakers go into lower power mode when not playing anything, and ‘wake up’ when playing.  So they are always drawing some current when not playing, but only a negligible amount.  This would apply to Sonos Amps which I am assuming are driving your in-ceiling speakers..  Obviously, the more speakers / amps, the more power consumption. 

Here’s information on Sonos power consumption when idle. 
 

https://support.sonos.com/en-ca/article/sonos-power-consumption-while-idle

While the system is in standby, power consumption is the same regardless of the number of speakers connected. It’s a more complex question to answer while the system is playing music. A first pass analysis suggests that if you are playing at a one Watt level, adding a second identical speaker will require an additional one Watt from the amplifier. Since amplifier efficiency varies with the power it is currently delivering, this additional one Watt of power delivered to the speakers will not require exactly one additional Watt from the power line.

That said, it is much more efficient to add a speaker to an existing amplifier, than to add an additional amplifier for that second speaker.