?-How many hearts for shadow box guest book?

So for our guest book we've decided to do the shadow box where guests can write a message and sign their name on the heart. We are inviting 115 people. I am assuming most couples would probably sign one heart together. Not sure if I should order 75 or 100 hearts? What are your thoughts? I don't want to have the shadow box too big and all this extra space because the hearts didn't get used.