The RSVP card should be printed on a separate piece of parchment along with a return envelope. Of course I’m assuming you really do want some idea of how many of those invited say they will be there. Statistically speaking there will be fewer show up than responded with a positive RSVP. Sometimes as high as a 50% difference.
The RSVP card:
____Yea I will gladly attend the betrothal
____ Nay, I regret that I shall not be attending