Anyone who does not send response cards is looking for financial disaster. No one knows if others have another obligation or if they will be out of town. In this economy it is a very, very foolish thing to do.
My rule of thumb is if a person doesn’t answer call them. Mail does get lost and it also goes to the wrong address. There has been many a friendship broken because people were insulted that they didn’t get an invitation. Don’t assume that people are lazy or inconsiderate (which is also the case) and didn’t answer. Make the call. Even for those who never answer, try to get hold of them. The problem with not chasing people down is they take someone else’s seat and you either wind up with people squishing in at the table or someone who is supposed to seat there looking for a seat elsewhere. But you can only do your best so make seats only for those that responded and those you know for sure will show up even if they didn’t respond (like your sister and mother).
It might not be classy but it might not be a bad idea to put alternate methods of RSVP on the return cards such as an email address or a phone number. Sometimes people do wait to the last minute and are embarrassed to call (and will pretend that they mailed it in buy oh my you didn’t get it?) but they won’t mind sending you an email because they don’t have to see you or hear you. An email two days before the wedding still allows you to accommodate them and it is much better than them taking someone else’s seat.
As far as how many people don’t show up that say they will. I would say less than 10%, but then again about 5% come when they didn’t answer they are coming. So unless it is really inclement weather about 5% of the expected guests won’t show up.
Of course if you invite the entire world to your simcha, you can expect a bigger let down. If they truly don’t have a real connection to you, anything will be enough of a reason not to show up.