Dave I think it really depends on the crowd you are serving. I noticed that older crowds tend to eat less while the young guns can pack it away. I've done cooks for the last three weekends one of which was with Larry R, actually two with Larry R. The first one I did was for thirty people and we cooked 4 racks of ribs, but we also had tri-tip and chicken to go with it. Most of the people there were over 50 so they went with the chicken and tri-tip, but didn't over do it by any means even thought they raved about the food. The following weekend Larry R and I did 12 racks for about 40 people maybe a little more. Now they were all mid twenties to early thirties in age, and got after it. We had pulled pork plus chicken to go along with the ribs. There wasn't too much left over except the pulled pork, and a half a rack of ribs that we (ok Larry...I) forgot to take off the grill, so that seemed to be just about right for that size crowd. This past weekend there were 6 adults and one child and we polished off two racks of ribs fairly easily. I think you'll be fine with six rack, but could probably get by with 5 racks. But if you buy 6 racks and they don't get eaten up, you'll have some for a late night snack. Either way you win...