We met up with a friend (so 3 adults total) and got a family bag of 10, and then each of us got supplemental food/drinks (for instance, I got chicory coffee and a side of bacon -- I don't mind sugarbombs for breakfast on special occasions but I knew I'd feel better if I had some protein). It ended up being a little too much; I think we did finish off the bag in the end (so in the neighborhood of 3 beignets each) but we probably would have been better off having just 2 apiece. If the teenage boys in your party fit the stereotype for appetite, maybe count them each as two adults and expect them to each eat four or five of the things.
I agree with the advice above, though, about just sitting down and eating them right after you buy them. We chose a day when the parks were opening at 9:00 to do our beignet breakfast precisely so we could show up at Ralph Brennan's at 8:00 and have time to sit down and eat. The line for food wasn't long at all, and even though we weren't concerned about needing to go back for more it was a good move in terms of being able to eat breakfast without ending up covered in powdered sugar. There's tons of it and it goes everywhere; the beignets are not good walking food, plus they're best when they're still hot.