With sides, common guidance is 1/3 lb meat per person. If it was me, two packer briskets and 15 racks of ribs (4 bones per person), but I always over cook for events. You'd be fine with one packer and 15 racks, but I NEVER want to have just enough food. The last thing I want to worry about is portion control. It's in my Italian blood. Send any leftovers home with folks in aluminum foil. Whenever I announce "who wants to take the leftovers home" you'd think I said "Free $100 bills here!" There's always such a rush.