If it was me cooking for that many people, I'd count on 40 folks. If your family reunions are anything like the ones we have, too much food is WAY better than not enough. Plus, people pick on the food long after it is served. They also take leftovers home if offered.
Next, how many adults, teenagers, age twenty-somethings, men, women, and children? Children don't each much, teenagers and twenty-somethings can inhale free food, adult men and women can be pickers or big eaters.
First off, how's your brisket? If you are satisfied with it and can repeat success each time, no problem. If you are new to brisket, you might want to switch to a pork butt and serve pulled pork. I'd do a 15 lb packer brisket, but only if it was requested. Brisket doesn't like to sit around after it is smoked. Make sure you have plenty of jus to keep it moist if you do a brisket.
With large groups I usually stick to pork butts instead of brisket. It's easier and ALWAYS comes out great. I'd do two 8-9 lb pork butts from Hormel or Smithfield. They are "enhanced" to ensure juiciness, tenderness, and flavor.
I'd do three or four racks of spare ribs. With 12-13 bones per rack, that would be plenty. Folks usually don't take but one or two ribs when there are several meat choices. Ribs are messier to eat and folks tend to eat less. Except kids, teenagers, and 20-somethings. Ribs are like crack to them.
If you really want to do chicken breast, I'd get two family packs of boneless, skinless chicken breasts, and brine them for 24 hours. Then I'd cut in each breast in half or thirds after they were smoked. Once again though, unless you're sold on chicken breast, boneless, skinless chicken thighs are much better, more forgiving, and don't require brining to be juicy and flavorful. A family pack of BSC thighs usually has 12-16 thighs in it. Three packs would be plenty and no need to cut them.
That is WAY too much meat actually, especially with sides, but people always graze after everyone has eaten.
Make smoking the food fun. If you have to work too hard or worry too much, you won't enjoy the gathering.
