For 500 people, you are looking at moving at least 175 pounds of finished meat and perhaps as much as 200 pounds. You would need to be looking at doing almost 300 pounds, or 35 to 40 butts for this, if all you do is pork. Can you realistically do this on site, and pre-event?
For me, with that many people, out of a temporary set-up, I would be looking at a simple menu, maybe two meats (my choice would be pulled pork and pulled chicken) as sandwiches, slaw, and something else, maybe roasted corn chunks. I would want to pre-cook as much as I could, but, if you don't have the facilities, then you will have to do it all on site. I see that as a problem with just one cooker.