Probably about 4 bags to get a thick coverage for something that large. Honestly for a space that big, unless you are super-dedicated to the idea of using the pellets, there are much cheaper alternatives. Bales of shavings take up more space, but they're lighter to handle and cost less - two or three bags will get you plenty of coverage for a 12x12 stall.
You'd have to use about 4-5 bags of the equine pelleted bedding for the same coverage. So to completely strip a stall and re-bed it would cost you $15 each time, versus a possible $30 in pellets. I guess it will depend on how much your horses are stalled, how messy they are, and if you have mats underneath them.
However, I have seen people say they bed with 7-8 bags of pellets to start with, and then just add a bag as needed. I use it for guinea pigs, and the males especially get really stinky, really quickly. It's the best thing I've found for controlling odor cheaply.
Mats are definitely worth it in bedding savings alone. Also, you won't have the gradual buildup of ammonia odors as pee soaks into the soil over long periods of time. You can combat it with quicklime (NOT CAUSTIC LIME), or a product like Sweet PDZ though.