It really depends on the stage designers choices. Like tstuart said, always bring slugs, just in case.
You want a shotgun that can hold 9 in the magazine, optimally. Normally, on a loaded+chambered start, that 9th round won't be needed (since you can only load to 8+1 if you aren't shooting Open). However, on a loaded & unchambered start, that 9th round can come in real handy... And makes it so you don't have to ghost load. (Division capacity is always 9 rounds regardless of loaded chamber or not)
I would recommend a minimum of 2 shotgun shell holders on your belt (or chest rig, if necessary). The easiest to learn and get fast with are probably the load 2 type. Here's an example:
http://www.makereadyproshop.com/taccom-duaload-8-up-shell-holder-12ga/ Note: These do not include the mounts, you must be those in addition to this!. You grab two, in line, and load them simultaneously. It requires a lot less practice and is generally faster than grabbing a handful and loading one at a time (the "old" method). The only downside is that the shell carriers are inexplicably expensive. You can probably easily sell them if you don't like them, however.
But if you want to save some money and try the sport out with some less expensive shell carriers, you can try these (which I have):
http://www.midwayusa.com/product/2394267180/california-competition-works-shell-caddy-tactical-shotshell-ammunition-carrier-12-gauge-6-round-2-3-4-and-3-shells-polymer. They work well but are considerably slower and more practice-intensive to get fast than the "load 2" type.
Hope this helps some!
