Just to give a couple of concrete examples:
Chicken with 40 cloves of garlic: a southern French classic, might serve 8 but probably more like 4, so in that dish, 80 cloves for 8 people.
Bourride: southern French classic seafood soup, normally accompanied with aioli, a sort of garlic mayonnaise. 6-8 cloves of garlic worked to a fair mayonnaise consistency with 2-3 egg yolks and a couple cups of good olive oil will serve 8 easily.
Bourride will be much, much more "garlicky" than the chicken with 40 cloves of garlic -- you'll smell it in the shower next day.
If you take whole heads of garlic, cut off the tip end, bathe in olive oil, wrap in foil, and bake until very soft, then spread on bread, I'd allow 1 head per 2-3 people.
And these are just southern French examples.
In short, there's no way to answer the question without knowing what you plan to do with the garlic, because different cooking methods produce such wildly different sorts of results.