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stiffy- it depends on how you plan on serving it. If doing sandwiches with a side dish, serve it on a roll type bun and plan on about 3 -4 ounces of meat per person. If serving as slices on a plate with two side dishes-figure 4-5 ounces.
Another thing for you to look at it is cooking times. Buying 4 large briskets will take much longer to cook than 6 small briskets. Try to keep them below 10 lb for a full packer and you should be fine. I have tried to do mass quantity of briskets for folks and picked up two packers each over 15 lb and then had troubles getting them done!!
BTW, what type of smoker are you using to cook the briskets? Do you have space for multiple briskets?