The wedding day is the most important day for a woman in her life. As a bride, she wants everything to be perfect on her wedding day from the theme of the wedding, wedding dress, hairstyle, jewelry to all the arrangements.
Number RSVP cardS and log who has which number on a spreadsheet. Easiest way to track who still owes you an RSVP. When you don't get them back (and there will be many that you won't, its just how it goes) you know exactly who to confirm with.