If this is a two week show, I'd plan for two or three bags initially, and a bag like every other day (depending how messy your horse is) (: I also know some people who use wood pellets (the horse bedding kind!) on the bottom layer and then add cedar or pine bedding above(:
Good luck and have fun!:) My best friend shows at the PtHA World every year, she does speed events and sometimes halter:) Her dad was president of the Central Oklahoma Pinto Club, but he passed away from cancer a few years back:(
To prevent people from feeding your horse, put all buckets to the back of the stall, don't put any buckets or hay bags on the front. This will keep your horse busy at the back of her stall away from the front where people could get to her, and also prevents people from dropping anything bad in her water.
Be sure to give her plenty of turn out time in the round pens (I'm from OKC, not sure if the Tulsa fair grounds have round pens like we do) every day so she doesn't get stiff and swollen.