Never done smashed taters for a catering gig but my guess would be the women won't be eating much of them so 4 gallons should be okay. May I assume you are going to serve the brisket both sliced and burnt ends? If so, your good on the sliced but will run out of burnt ends. I would probably go with 100 thighs, better to leave them with leftovers than to run out and thighs are pretty cheap.
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