Well, it seems to change every year now. When I was there, we had 3 seperate teams all vying for different points on the field. Certain bunkers were worth points if you held them for a certain amount of time, and at the end of the two days, highest points wins!! You can go for one day, or two days (We did one). You get there, register, get on a team and then the battle starts. There are no different fields, just one huge one, and you can either reinsert yourself into the battle after being shot (After walking back to the MASH), or you can hop on a troop transport (At MASH) and get reinserted on the other side of field somewhere (Or evil option three, when you're on the FAR side and you get knocked out, you walk halfway back, and reinsert yourself with your team in a different battle). Its was basically all out war when I was there, and sooo much fun. You could walk around the woods by yourself, snipe, go with teammates, capture the bunkers, you name it, you could do it. There was also a mercenary team that could get hired depending on who wanted to shell out the most allotted fake money. I'd do it again if I ever had the time anymore.
I should clarify though, the 1200 number is total number of players over the whole weekend, not at one time. Still, there are TONS of guys out there.