You send out your wedding invitations at least 3 months before the wedding with an enclosed card/envelope (with stamp on it) so the guest you are asking will tell you if it will be just them alone or they are bringing their partner. It is up to either the mother of the bride or the Maid (Matron) of Honor to contact those guests asked that have not replied. Once you have all names of the guests coming to the wedding that is when you tell the caterer how many people are coming. Beware! Some people may say they are coming and for reasons of their own can't make it, but a buffet is a good way to go instead of individual place settings for a wedding. Individual place settings for wedding guests can range from over $25 and up depending on where the reception is being held.