Thanks, sorry for the delay in responding.
As I did not have anything to do with the design, speaker placement etc, I can only work with what I have been given.
Initially, without documents, I was told that there were around 20 zones of music.
It is a very large home, but as many have pointed out, it does not need to be able to play 20 different things in 20 different rooms. This is more about being able to select from maybe 6 or 8 sources (His, Hers, kids, Guest, Patio... whatever. I will have to figure out naming with the client) and turn on, control volume in 20 - now 30 areas of the house. As we have no option of grouping areas and just adding volume controls, we are dealing with 30 "zones" There are at least 41 or 42 pairs of speakers throughout.
Initially, my thought was to use 20 Sonos Amps and call it a day. Not for the purpose of having something different playing everywhere, but for just the ease of set up and use.
When the client walked through with us and we discovered we were up to 30 zones, and an extra $10K was not really in the budget, I figured there would be another way to do this. I also saw a few comments about using Sonos closer to it's limits can start to have problems.

I like the idea of Yamaha as there is an RTI driver, and there are two Surround systems that I could use Yamaha AVR's in and have music cast in those areas too.
Problem,I was first told was Yamaha does not work well past 10 zones, although the Yamaha pieces mentioned above do say up to 32 zones.