Been there, done that . You don't need to waste time and energy worrying about the number of people who attend your reunion. You are feeling like the weight of the world is on your shoulders because you have the responsibility of signing the contract. Well, the real deal is that people will attend the reunion and enjoy themselves no matter how large or small the event. Yes, back in the day, I organized a few events that didn't have quite the turnout that I had hoped for, snafus galore, and yes, some family drama, but somehow I managed to just enjoy "the moment".
My best advice(not that you asked for it) is to not waste time and energy trying to anticipate what people will do and what might go wrong. If they attend the reunion, cool. If not, it's still good, because you're still going to have fun. Remember to enjoy "the moment" and use the reunion planning process as a learning experience.