Very few couples are lucky enough to predict the exact number of guests that will attend their wedding and reception. The best they can do is give a reasonably accurate estimate to their caterer and keep their fingers crossed. Even with careful planning, printed RSVP cards and lots of online communication, the best most weddings can manage is an 85% rate of response. What's a bride to do? As awkward as it sounds, many brides prepare a B-list of guests.
Those couples who really want to nail down the exact number, begin to plan the guest list early in their engagement process. It's important to begin thinking about your guest list and who to include on the "A" list, the names of those guests who absolutely must be invited. The wedding wouldn't be the same without them. Once that list is complete, many brides are then assembling a list of potential guests whom they would love to invite if only they had the room. Many wedding planners suggest their brides begin building an alternative list once the primary list is complete. The suggestion is to order extra invitations with the originals and have them ready to go when the RSVP's begin to arrive.
It may mean adjusting the invitation mailing a bit - many suggest mailing A-list invitations slightly ahead of the 2 month mark but keeping a one month RSVP date.
Keep that B-list your secret. There's no need to broadcast its existence. You and your mom may know - and the groom, but no one else needs to know. And if possible, keep B-list guests as a group. If one guest from work is invited, place the others on the same schedule.