Is your plan to get 12 of the AirPort Expresses? Or just one and share it across all 12 rooms?
The recommendation for an amplifier, from me, would be at the very least a 12 channel amplifier via eBay similar to this:
Niles SI-1230 12 Channel Amplifier (760514007376) | eBay
If your intent is to have 12 separate zones with the potential for different audio, then you will need to explain more of what that goal is.
With that amplifier and one Airport, you can send audio out to 12 rooms. Once source, 12 matching, identical, destinations.
If you want more sources, and more flexibility, then you will need an audio matrix switcher ahead of at least 24 channels of amplification, which works fine, but will cost a bit more.