"How many invitations do you think you need?" The very first question to clients calling to place an order. And interestingly enough ... most clients give me the number of guests they are estimating. Ah ha! But this is not how many invitations you need. You'll need about 1/2 your guest list, plus some, plus a few extras. My oh, so very sophisticated formula!
Here's a sample - say 200 guests, figure 100 (this is the 1/2 - remember you'll have some couples and families), plus about 10% (this is the plus some - for those singles) and a minimum of 10 extra (I usually recommend 15-20 extra - guests (oops!) you may have forgotten, lost invites, and new friends). That would be around 130-150 invitations.