I would do 2 butts for that many people . You will lose about 20 to 25 percent due to fat and bone . I do not inject. Only use rub . You want to get to a internal temp of 190 . Depending on pit temp will determine how long . I wrap my butts in foil for last 4 hours of cook . As for mopping or spritz i would not do . Someone always told me " Looking is Not Cooking " Very true . The more you open your pit the more temp goes up and down .
2- 22.5 WSM , 150 Gallon Offset , 250 Gallon Reverse Flow