Once you've started your wedding guest list, and gathered names, separate your list into out-of-town and local.
Local guestsIf you send your invitations out 6 to 8 weeks in advance, the likelihood is that most of them will come. So estimate that 85% of them will attend. Don't forget to account for plus ones, if you've invited them.
Out-of-town guestsThis depends slightly on the relative wealth of your out-of-town guests and how many of them are family. Got a wealthy crowd that's mostly family? Estimate that 85% of them will attend. But if most of your out-of-town guests are old college buddies that you haven't seen in years, your acceptance rate is probably closer to 40%. For most couples, a safe estimate of acceptance rate for out-of-town guests is 55%
Absolute MaximumsIf you've got a ceremony that will absolutely not seat more than 60 people, then you should be a bit more conservative than the numbers above.
Once you've got your decline rates figured, add them together:
(# of out of town guests * 65%) + (# of local guests * 90%) = total # estimated to attend.
So if you have 150 people on your guest list, 50 of which are out-of-towners, and 100 are local, your equation will look like this:
(50*65%)+ (100*90%)= 122.5.
So, approximately 123 people will attend.
Please note, as I said, this is an art, not a science. So it's a good idea to make an a list and a b list. Send your a list invitations out 8 weeks in advance. As you start to get acceptances and declines, you can start inviting some of the people from your b list.
Read more about starting your wedding guest list.