How many baby backs and brisket for 20 mixed adults?

For a mixed group you can figure 3-4 people per Rack of Ribs and 5-6 ounces of Smoked Brisket. 3oz of Beans and Potato Salad should be more than enough.

Since you are serving Rib and Brisket, when it comes to ribs people will typically eat 3-4 per person. 4 ribs per X 20 people = 80 ribs / 12 ribs per rack = 6.7 go with 7 Racks of Baby Backs. 

Since Brisket has about a 50% yield...6oz X 20 servings = 120oz X 2 for yield = 240oz / 16 = 15Lbs raw Brisket needs to be Smoked.

3oz X 20 = 60oz / 16 = 3.75 Pounds of each Side Dish

Some will eat the Brisket just on the plate others will make a sandwich if Bread or Rolls are available. I have weighted it toward the bigger eaters so you may very well have some leftovers but better too much than not enough. Good Luck...JJ