You can only connect six speakers to a single Sonos Amp if the speakers are Sonos Architectural by Sonance speakers. If they are not, you can only connect four speakers.

It’s not the wattage you need to be concerned about, but the ohms.

https://support.sonos.com/en-us/article/connect-four-speakers-to-your-amp-or-connect-amp

The short answer is yes. But what you will get is better coverage of sound over a large listening space, but make sure you follow recommended Sonos practice for more speakers than just the usual pair.

So the volume in each speaker will not vary if I connect either 2 speakers of 8ohms or 6 speakers of 8ohms to a 250watt 8ohm amp? 

You cannot connect 6 speakers of 8 Ohms unless they are the aforementioned “made for Sonos” Sonance speakers.  The limit is 2 pairs of 8 Ohms or 1 pair of 4 Ohms.  And yes, the overall maximum volume from each pair will drop with each additional pair.

If this was a simple lamp circuit we’d keep adding lamps until we reached the maximum current supported by the circuit. At a first approximation lamp output will not change as we add more lamps to the circuit. Lamp circuits operate at a constant voltage.

Speaker “Impedance” varies with frequency. When we specify “8-Ohms” we mean the “magnitude” of the impedance and the current drawn if we replace the speaker with an 8-Ohm resistor. Resistors don’t change their value with frequency. This implies that, at a fixed amplifier Volume (voltage) setting, as we add more speakers the output of each speaker will not change and we can continue to add more speakers until the current limit of the amplifier is reached. 

In the case of a SONOS AMP, we can reach the current rating when two pairs of 8-Ohm speakers are attached and operating at AMP’s rated output voltage. (and the frequency is at the 8-Ohm point)

Unfortunately, there is no industry standard with respect to assigning an impedance number to a speaker. Different manufacturers might assign a different number to the same speaker. Usually, the assigned number is near the low point of the magnitude of the impedance and this frequency is usually in the 50Hz range. In some notorious speakers the magnitude of the impedance might be lower than 8-Ohms at some lower frequency, say 20Hz. Some amplifiers will gracefully deal with this as long as there are not too many sustained 20Hz notes (causing the amplifier to overheat). Other amplifiers will freak out if there is a momentary dip below their minimum impedance rating. AMP is graceful.