There is no one "right" way to write a response card. There are many different styles to choose from. It can be as easy as, "We look forward to celebrating with you. Please reply by (date)." After that, you would leave a line for the name and perhaps a checkbox for can or cannot make it.'
Don't expect everyone to reply back to you with the card that you sent them. Some might e-mail or call you directly. You can't expect everyone to write legibly on the cards so it is best that you write a list of people with a number next to each. After you write a list, you can number each card so even if you can't read the name, you can still figure out who is who by looking at the number.
Your guests might invite their friends and family along so make sure you write somewhere on the card "Number of guests?" Never assume that the person will come alone.
Make sure you make it very easy for your guests to reply, such as leaving your phone number and e-mail address on the card, and an envelope with a stamp on it.