Is this a vending gig where you set up and people buy sandwiches from you?
How many other vendors? How many people attended the event last year? Who cooked last year? Why is the old vendor not coming back? If this is a first year event it is a gamble. Organizer's expectations are usually not met.
How many hours does the event last? Is the event being held during normal meal times? Is this a one day event? Is this a charity that you would like to support?
These questions need to be answered in order to figure out how many people that you are going to feed. Do you have a place to sell any meat that might be left over?
Burgers and hot dogs cook in minutes from frozen product. Pulled pork is labor intensive in comparison and takes a long time to cook.
You can approach this 2 ways---#1 make pulled pork for 300 sandwiches and 300 buns and close if you sell out.
or #2 -cook enough pulled pork for 750 people and get 750 buns and take home leftovers instead of money--or maybe get lucky and go home with lotta money.
I would do more research about the event and increase your odds of making money