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I usually do 1/2lb per person.. But I always overcook so I have leftovers. Especially shoulder so i have some next time I make beans. I think normal guidelines would be 1/4lb per person? Cant hurt to have more :)
I think the general rule of thumb is about a 1/4# finished per person with sides. There's about 40% loss from raw to smoked butt I believe. So given 12 people assuming that their all adults you'll need roughly 3+ pounds of finished product. I would say an 8lber would do, but having a little extra is always safe. Someone will chime in soon and let you know if my numbers don't add up.