Ten people are coming on Saturday. The real question is how many bottles to take out of the rack, so that you are neither short halfway through the evening nor left with four opened bottles on Sunday morning.
The arithmetic, in one line
A 750 ml bottle holds five generous glasses of 150 ml, or six at 125 ml. For reference, a standard drink in the United States is 5 fluid ounces of wine at 12 percent alcohol, which is close to 150 ml, as the CDC sets out. One bottle is therefore about five standard drinks, which makes the planning number easy to hold in your head.
For a seated dinner of two to three hours, half a bottle per drinking guest is the usual starting point: roughly one glass in the first hour and a little less in each hour after. A standing party where wine is the only drink runs higher. A long lunch with children present runs much lower.
Do the count on paper before you shop
Guests: Of those, drinking wine: Hours at the table: Glasses per drinker, first hour then about three quarters after: Total glasses: Bottles needed, glasses divided by five: Rounded up, plus one spare: Split: sparkling / white / red:
The spare is not padding. It covers the bottle that turns out to be faulty, the guest who only drinks white, and the conversation that runs an hour longer than planned. An unopened spare goes back on the shelf. A missing bottle ends the evening early.
Split by role, not by guessing preferences
Trying to predict who drinks what produces a strange shopping list. Splitting by role is easier and more reliable: something sparkling for the first half hour, a white for the starter and for the guests who stay with it, reds for the main course, and one bottle that does not need to match anything.
If you are choosing from your own collection rather than shopping, the shortlist method works here too. Pick three candidates per role, check what is left of each, and choose the one you can spare rather than the one with the best label.
Set the bottles aside before the day
Take the wine out of the rack the night before, or at least mark it as reserved. A bottle you planned for Saturday is very easy to open on Thursday, especially in a household where more than one person takes wine off the shelf. The two-person cellar guide covers marking a reservation so it does not read as an opening.
Bring reds to the temperature of the room a few hours ahead, and chill more white and sparkling than you think you need. A warm bottle can be cooled in twenty minutes. A cold one takes an hour to come round, and nobody waits.
What to do with what you did not open
Unopened bottles go back to their locations. Correct the record if they moved shelves, because a bottle in the kitchen after a party is exactly the kind of bottle that gets counted as missing three months later.
For the bottles that were opened and not finished, the leftover guide covers what keeps, for how long, and how to record the opening date so the next person knows what they are pouring.
Plan for the people who are not drinking
Some guests do not drink, some are driving, and some will have one glass out of politeness. Counting every head as a wine drinker is the most common way to end up with a rack of half-finished bottles. Ask, or plan for fewer drinkers than guests, and put as much thought into what else is on the table.