Or how many say they will show up, but then (for one reason or another) don’t?
It seems like there ought to be a standard answer, doesn't it? Surely between the bride's friends and the groom's friends, between Aunt Martha (who would never, ever dream of accepting an invitation and then not showing up) and Uncle Jerry (whose actions seem to be purely random), it ought to all average out to some neat, round number.
The bad news is that it doesn't work like that.
With 120 or so games a season, statistics work for baseball - but a baseball game always has nine players to a side (ten if there's a designated hitter) and the players always have to cover the same positions and play by the same rules. With 120 or so guests (or even more), statistics don't work for wedding invitations.
There are two reasons for this. One is the obvious one, that no two guest lists have to cover the same positions, much less play by the same rules. If you belong to the sort of family where second cousins will put the kids in the car and drive 900 miles for any sort of family event, you'll get very different guest list results than a bride whose siblings don't bother to remember each others' birthdays.
The second reason is that the cost of being on the wrong side of the odds is much higher with wedding receptions than with baseball. If a .340 batting average becomes, after a bad day, a .295 batting average, the player may lose some incentive money, but life pretty much goes on as before. If you estimate low on the number of plates you need at the reception, someone won't get fed. And that upsets people who put the kids in the car and drove 900 miles to be at your wedding.
So what do you do?
First, it helps to realize that no matter how carefully you plan, you can't guarantee that some family won't have a cancelled flight or a ghastly car accident on the way to the wedding, leaving you with four uneaten meals to pay for. It happens. Your goal is not to control every detail, but to minimize the amount of stress and disruption that can be caused by fate, flakiness, or just plain rudeness.
Start by matching the number of people you can afford to entertain with the names of people who would be sadly missed if they weren't present. It's best to make the list of potential guests first, and then figure out how to entertain them. However, plenty of couples and families start by reserving a hall and then cut the guest list to fit. Doing it the second way is a Procrustean experience and painful for everyone, but most weddings survive it. Always give first priority to the guests who are most important to the bride and groom, with some additional weight given to family ties.
The number of people invited in the first round of invitations should be less than or equal to the maximum number of people you can afford to feed. If you can afford to do something "nice" and invite everyone that it would be pleasant to include --- and don't forget that a mid-afternoon cold-cut tray can be "nice" if the wedding is relatively less formal --- then one round of invitations is all you send, and life is good. That alone may be a case for considering less formal weddings! The price of the bride's wearing a dress with a 12-foot train for five hours is often months of arguing and tears over the guest list. (The more I hear about wedding problems, the more I think there's something to be said for simple weddings and really fabulous anniversary parties.)
If the budget didn't stretch to entertaining, in a style you find comfortable, every person that it would be nice to invite, then you need a "B list." This is a list of people who will be invited if someone else declines. The B-list system works best if the first round of invitations is sent 8 weeks before the wedding, so that B-list guests are likely to receive their invitations about four weeks before the wedding. It's also better if B-list guests live relatively near the wedding site, as it's often impossible to get an affordable plane fare without a 21-day advance purchase.<
This system assumes, of course, that people who wish to decline will actually respond to the invitation! Not everyone has manners any more. It is not improper to drop hints, in your talks with or letters to guests, that a timely response is important. And you should recruit other contributors to the guest list to do the same with their friends. As the last possible day to respond approaches, it's also quite proper to phone non-responding guests to ask whether they intend to attend.
Once you're into the week before the wedding, the proper answer to people who tell you they still don't know is usually "So sorry we won't see you there." (A compassionate exception is appropriate for expectant mothers in their ninth month, for emergency rescue workers, and the like.)
Can you still invite B-list guests at the last minute? Yes, but you put yourself and them in a delicate situation. Limit such invitations to nearby people who are known to be easy-going. Be honest -- be gushy, even -- about how you wanted to invite them but there was a budget crunch, and you're thrilled that a spot opened up at the last minute. And don't even try such an invitation if any of the key players in the wedding believes the myth that a wedding guest's gift must cover the cost of his dinner! People who are invited the day before the wedding may not rush to the nearest crystal display and whip out their charge cards.
As for the no-shows who accepted, you can decide later, when you hear their excuses, whether you ever want to speak to them again. The advantage of having limited the count to what you could afford is that, though their meal might be wasted, it wasn't waste that went beyond your budget. And it sometimes happens that the missing persons pretty much compensate for the people who were hauled along without your permission, leaving the whole thing even as long as you have a sense of humor about the unexpected.