Every time we get a question about "how much X should I make for N people?" we go through the same dance with clarifications: what's the full menu? what kind of people are you serving to? Then we tend to supply a vague answer, with details depending on who took the time to answer on this occasion.
Is it feasible to write a generic question to cover some of the main things here? Some of the most common things are meat/fish or starches as main dishes, and various sides. Presumably we'd have ranges, to cover the gamut from not-too-hungry mixed crowds to ravenous teenagers after a hike.
It seems reasonably possible, since there are rough tables for this kind of thing all over the internet. And if we had it, we could redirect plenty of questions to it, sometimes as-is, sometimes with little notes about adjustments for the OP's specific situation.
I'd certainly be happy to hand out a bounty for a really solid answer!