For 500 people and really, once they are 14, they are adults. I would consider that hot dogs are a minimal item, however, hot dogs store well. Burgers, unless you are doing fresh, can be frozen and thus, they store well also. I see the breakdown as thus.
500 total people, two servings per person
350 adult portions (assume 2/3 take a brat and burger, 1/3 hot dog)
200 children (assume hot dogs, singles)
315 hot dogs
But, as stated above, you can freeze most of this stuff, and in these amounts, you are going to have the ability to buy wholesale at low prices. Over buy, this is not the thing you need to skimp on. I would buy...
350 hot dogs
BTW, don't forget that you will need a lot of condiments, and it is better to buy them in large quantities and repackage into several small squeeze bottles, if possible. You cut down on both waste and spoilage that way, plus, you can move the line faster with several small condiment locations.
I'm feeling bearish, and I'm packing a Wusthof Grand Brisket slicer from MABA
"perhaps...but then again...maybe not..."