When I smoke I usually figure 7.5 lbs of cooked meat for 20 people and I usually have some left because not everyone eats brisket. With lots of other food you can probably get away with maybe 10 lbs. But that's just a guess because I don't know you or your family. Have fun at your reunion.
I'd say 10 to 15lbs
20 should be good enough given many others provided
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