I think R.S.V.P's are a great, but tricky thing.
R.S.V.P. stands for a French phrase, "répondez, s'il vous plaît," which means "please reply." The person sending the invitation would like you to tell him or her whether you accept or decline the invitation. That is, will you be coming to the event or not? I don't think many people know exactly what RSVP stands for, so they do nothing. Most response cards, RSVP, or rely cards look like this,

Hoping the guest will add their name and if they will attend. I have learned over time that some guest don't put their name in the blank and will just send it back. In that case, I always get my hands on my clients reply cards and number them before they send out. Then I number the guest list to match the reply cards, so in case a guest doesn't fill in the blank, I know exactly who it is. Just a little trick I do, and it works every time.
Or you may be left calling all of your guest, asking them if they are coming to your wedding.
It's also good to make the date a week or two earlier than when you have to give the final guest count to the caterer.