Does there happen to be a line between the words will and attend? If so, it's meant for you to write the word not if you aren't able to attend. By leaving it blank and returning it, the host assumes you will attend and that the number of people attending is equivalent to the addressees on the (inner) envelope. For example, the host will expect 2 guests if addressed to Mr. and Mrs., or Mr. and Guest, 4 if addressed to Mr. and Mrs. and family, where family includes two kids, or adressed to John, Jane, Joe and Jessica on the inner envelope.

You write either:

"Mr. and Mrs. John Smith will be delighted to attend. We look forward to it!"

OR

"Mr. and Mrs. John Smith will regretfully be unable to attend. Best wishes!"

Just write, a note on the card that says you won;t be able to attend.

Or put a 0 or write Regrets in the number of attendees?

sorry this is a dumb question. I received an invitation to a wedding next month with an rsvp card in it. The card just says "We look forward to celebrating with you" and a place for our name and number of attendees. There is no place for "We will be unable to attend". Do you just send this back if you are going, and your lack of reply is all that is needed if you are not going? Or do I write "we will be unable to attend" in the place provided for number of guests?