I'm with tropics on this one, Leftovers isn't a bad thing but folks being hungry SUCKS. And you never know if "Uncle Joe", "Aunt Barbara" and some siblings may show up!
Bring some Zip-locs!
Make the burgers on the smaller sides and more of them, nothing like seeing a 1/3rd pound burger in the trash with a bite out of it.
I have learned over the years to expect the worst and hope for the best, and have lost some respect for the human race, I have seen pizza parties where a kid would grab 3 slices of pizza, eat half of the three slices and go back for more when there's still kids in line for their first piece and the parents see this. Gluttony really gets my goat!
You have a small crowd and this is probably not a problem...
Will you be serving or will you be putting everything out for folks to come up and take?
If you are serving, you have control over this, if it is a buffet style, that may be different.
If money isn't an issue, Overcook and take home the leftovers or send home some care packages with the other adults.
Believe it or not averaging is easier with larger numbers than smaller numbers, for meal planning, to an extent anyway, IMHO.
