Reply cards, such as the Starfish Border RSVP shown, can be worded simply with the main goal being a correct count of guests who will be at your wedding. Guests will add their name or names somewhere on the card and indicate whether they accept or decline the invitation. Any wording can be used and it’s really up to the couple as to how formal they want to be.
It’s a little trickier to make guests commit, on the RSVP, to a specific number planning to attend, but this may be necessary. Asking for a “guest count” or “number attending” is one way to collect a specific number. It’s important to have enough food and drink at the reception.
I have a page that was recently written and it covers various types of wording for the reply cards. You can see it here: Wedding Reply Card Wording Ideas. It covers the basics and can be applied to any wedding theme.
Also, you may be interested in this blog post: Alternative Ideas for Responding to a Wedding Invitation