No one likes to go stag to a wedding, especially if they are going to be sitting at a table with a bunch of couples that they don't even know.
When Marc and Trish were married a little more than a year ago, they made sure to include a "plus one" for each guest that was married or in a serious long-term relationship. Marc added, "We also considered whether the individual knew others who would be there and made sure they wouldn't be alone." While there wound up being several singles at the wedding, everyone was part of a group and the bridal couple planned their seating chart to allow everyone to sit with people they knew or had things in common with.
If you read wedding advice sites and books, most of the suggestions are to invite all married guests with their spouse and to address the invitation as such so there is no confusion; use the same rules for singles in a long-term relationship (at least three to six months) or cohabiting. Depending on the size of your celebration and availability of seats, you can also include plus one invitations for adult extended family and very close friends. You are not under obligation to invite plus ones for teens, single distant cousins that you haven't seen or talked with for years, or that random person you don't even know that someone in your family talked you into inviting. However if it is just a few adults who would otherwise be solo, you might consider including guest spots for each, but only if your budget and the venue size permits. You can, if you choose, give extra consideration to allow dates for your younger siblings or for junior bridesmaids.
If the invitee's significant other genuinely cannot make the wedding you are not under obligation to leave the invitation open-ended, this is one of the reasons it is best to include the plus one on the invitation by name or title. There are always some guests who will try to wrangle an extra spot or even modify the RSVP to include a guest. Stay strong and explain politely that you only have so many seats and you are trying to be fair to everyone by sticking to a consistent rule, and then smile and let them know how excited you are that he or she will be present for the occasion. While your budget may indeed be a factor it is a good idea not to use that as an excuse since your guest may offer to pay for the extra seat and other guests invite without a companion may feel slighted. By the way, your consistent rule is still OK if you allow your close adult first cousin more leniency than the teenage son of your father's third cousin twice removed.
*What do you do if the unexpected guest shows up the day of the affair?
If the guest is a plus one accompanying someone you genuinely invited and wanted to attend, it is probably easiest to just accept this last-minute change of plans instead of making a scene and ruining your day. Speak to members of your bridal party beforehand to be on the lookout for extra guests (a good clue is to watch for people who do not find their names on seating place cards) and have them contact the correct venue manager to place and extra seat at a table if there is room.
Placing an uninvited plus one at an already full table might make it uncomfortable for your other guests, leaving a copy of your seating chart with your bridal party member makes it easy to identify available seats so the couple can be moved to another table. Ideally, you should not move invited guests to other tables to accommodate the unplanned plus one. If necessary many venues can often set an additional table up for overflows although there probably will be an extra charge. Despite your careful placement of each guest and the time you took trying to seat everyone with enjoyable company, don't let it bother you if the couple with the unexpected plus one is now seated with strangers that they have nothing in common with.