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Personally I like to figure 1/3 lb per person finished weight and you'll yield 50-55% of raw weight.
22 make 24 it's easier 24+3= 8 pounds of finished product so start with 16 lbs of bone in butt and you should be fine. That is figuring you'll have sides to go with the meal. I would suggest a couple batches of Dutch's wicked beans as one side