I don't know how you will get that 12-way screen to happen from just one AppleTV.
I did find this
16-port HDMI splitter on Amazon
But, it mirrors the original image to as many as 16 outputs,
not 16 pieces (or even 12) to make a large video wall.
You will need to get a video controller that will do that - either in software (not on OS X, but I could be wrong on that), or a hardware solution of some kind.
What is your budget? It's not gonna be the "cheap box from Amazon" that you are hoping for.
You need a controller that will compensate for differences in color and brightness for all those displays, and also allows for the "gaps" that will affect multi-screen displays, etc. (You need to spend a lot of cash to get multiple identical displays that all display close to identically. It will be painfully obvious on a 12-screen wall.
Look at some of the sites that sell and support display walls.
You may even have a pro AV company local to you, where you can ask questions, and get answers that will be useful to you.