OK here is what I recommend and use...
Yamaha aw190 out doors... {if you are doing outdoor zones...} my out doors have these
Yamaha NS-IW280C - $75 each set of 2 Shop around, I have this exact model, and they sound great, plus they are very easy to put in ... And last a long time...
I also recommend doing 2- 6 channels {which is 12 speakers} for each zone using 2 of these
6-Channel Speaker Selector | eBay
Then use Jamo AT-A1 volume controls, these are not Impedence matching but the monoprice selector box is so you wont hurt your amps. I used to have the imped matching volume controls but they were terrible to deal with always breaking and going up and down in volume..
16ga wire will do fine and you will need a lot, I would get 16-4 and 16-2.... The 16-4 runs from each amp to each selector box, then from the box to the volume controls, the 16-2 runs from the vol control to each speaker, pretty simple..
For amps, I recommend 2 Dayton APA150's...
So here is how it works... You want 24 speakers which will be 12 zones of stereo {2 each zone}. So you need
12 sets of speakers $75 per set $900 total ebay has the best deal on the yamahas
2 6 zone selector boxes with imped matching $50 ebay again...
2 Dayton amps $160 each so $320 ebay again
12 Jamo at a1 vol controls 20 each, you guessed it ebay $240
total price- $1510.00 before you buy the wire...
Now onto how you are going to split it up into zones... I use zone 2 off of my Home theaters preamp, and run 1 source for all of my zones, since we ussually use it when we have parties or my wife is cleaning, one source is fine for use, with the setup above you can do 2 sources if you want more you need more amps, and more smaller selector boxes...
I have an Ipod wall system, iport iw20 but I can play anything that is plugged into my preamp throughout the entire house and out side...
The daytons are perfect for this type of system and the yamahas scream with that little bit of power... Multi room sysem are meant for ambiance not really loud monitoring but running 12 of them speakers off of the dayton they can still go pretty loud...
A more expensive way of doing it is a multiroom amp {I believe they have 12 channels and down} but In my experience it doesnt pay off, you run a lot more wires, and the amps are expensive and problematic, I replaced mine a couple years ago with this type of system, the amp broke twice under warranty, and then the last time it was not covered, one entire side is out... and it cost $1500 {thats what you can do an entire system for...}
If you do want 12 separate zones, I would go with 12 daytons and then just use the volume controls direct with no selector box. It would be cable of being super loud, take up a lot of room and power though... then use a a couple audio selector boxes to all of the amps... Then each room would have its own separate system that could be all ran at the same time, I would say this adds... $2000 to the 1500, so $3500 would still be better than buying 2 $1500 12 channel amps...
Good luck feel free to ask any questions...