Yeah, HarborMaster's right on....that usually gets everyone a goodly amount.......
there's catering calculators floating around the web that can get you pretty precise for one meat, two meats, # of sides, etc.
Mine are sort of odd cookups in that I'll do a batch of stuff overnite at home, guessing at 35 people, maybe........& have some 'Turds & other things ready to go, or done & hot holding........
Then, hot holding, transport across town to my friend's house w/ the big back yard..........
One time, 85 showed up.....glad I went heavy on stuff!!!!
Then other people will do more sides & sometimes folks bring things like chicken pieces, sausages, pre-made tamales, stuff outta the garden, marinated flank, etc.
Sometimes we just eat 'till it's gone, sometimes send some care packages to folks who couldn't make it by, I'll freeze some lefties for bean pots & other things.....
These cookups I've done at these friend's house every possible last Sunday of the month for years is how I really learned flexibility in cooking BBQ......no way to get an exact head count, so lots of change-ups can happen.....lite more KETTLES!!!!