Okay, I'll venture a serious reply now.
Without more specific information on what you are serving, how it is being served, etc here is what I would suggest.
Not everyone uses sauce. In my experience, in my area anyway, only about 50% use sauce. And the average usage for our customers is approx 3 oz per sammie. Therefore I would plan on a minimum of 3 gallons of sauce.
((200/2) * 3 oz)/128 oz = 2.34 gals
We do an annual event that serves right at 300 customers with pulled pork/brisket sammies and we have always taken 3 gallons (it is self serve on the sauce end). We have never run out but have always had to open the third gallon near the end of the event.
Your mileage may vary but remember that it is always better to have extra and not need it than to run out.
Hope this helps.