With a targeted invitation, you can count on a turnout of 40 to 60 per cent of your invitees. If you invite your top relations personally, it's closer to 70 to 80 per cent.
Always send 40 per cent more invitations than the number of places you want to fill. Call invitees who didn't respond personally. That usually saves 5 to 10 per cent of empty places on the day itself.