Like Smokin says,it all depends.
Seems like some folks that cater mixed crowds use 2/3 lb raw weight packer per person,if there is one meat, and toss in 10 % extra for walkins,etc to be sure.
So roughly:
70 lb X 67% =47 lbs+ [10%] 5 lbs.= 52 lbs.
That would be less than a light case, at a box store,which might run 60 lbs.
If case price is $0.20/lb less than singles,I'd buy the case at about the same cost.
You'd be covered for sure,and everybody loves doggie bags.
Well,almost everybody.
My spouse/co-cook says we have done so many practice cooks that the neighbors hide, like it is squash season.