If light refreshments are planned for a two-hour function, you’ll need about a half-bottle for every person, understanding that very few will consume as much as a half-bottle. Many will try two or three different wines until they find one they like.
Whites and rosés tend to be favored, especially for daytime al fresco events. Chardonnay is almost mandatory. And it needn’t be Le Montrachet. In fact, most newcomers would spit such a wine into the potted palm.