One of the worst social or business etiquette blunders is not answering an RSVP. The person who sent the invitation needs to know how many people to plan for.
What should you do if you sent invitations with an RSVP and not all of your invites responded? Contact the individuals to get an exact count for your event You should not assume the individuals who did not respond will not attend. Doing so may place you in a most awkward position if they unexpectedly show up.