How many more people usually come to a party who have not rsvped?!
If they have not responded, then give them a quick call to get a definite answer>>explain you need an accurate head count because of food and beverages. If they tell you they're not coming, just be gracious and say "ohhh, I'm sorry to hear that...maybe another time?" and let it go. Don't force anyone...if they miss out, they miss out.
It really depends on the people. Some people RSVP that they are coming but than they don't show up.