As you are planning for your big day you need to know how many guest to plan for so the RSVP or response card is very important. Things that hinge on the number of guest include: How many tables and chairs you will need, how much food and beverage you will need, how large a cake you will need, how many centerpieces, and how many wedding favors. Just to name a few.
Helpful hints to the guest: Please send this card back. The couple really wants to know if your coming or not. They will be okay if you have to respectfully decline. Don't assume that by not sending it in they know you can't make it. Also don't assume that because you mentioned you would be there they remembered.
Also put your name on the card somewhere to make sure they know who is replying.
Most couples have already addressed and stamped the card its not hard to send it back in the mail.
To the couples: Discreetly put a number on each card and make sure that number relates to the guest list. For example. Aunt Mary is number 14 on your guest list in a discreet corner write 14 on the card or in the inside of the envelope. So when you get card 14 back and Aunt Mary forgot to write her name you know who sent it.
Also if it fits your wedding you can do a response post card. The postage on a postcard is cheaper.
Make it clean what you need to know on the response card. You can look on line and get plenty of ideas on wording depending on the details of your day.
Know that even if you did not write "plus one" your guest will still try to bring extra guests. You will need to be prepared with a response. Either plan for some extras or plan to call then and let them know you did not plan on anyone extra.
Give guest about three weeks prior to your wedding as a deadline. This gives them time to make arrangements and confirm their calendars.