I've been on cruises before - but at least we have that many people that know about it and have expressed a strong expression of their interest, enough to say that they want to come. The more we have saying they want to come, in theory, means the more attendees we have. Even if we lose 30 we still have a very decent amount.
And it's not exactly like we need 200 cars on the cruise, as long as we have a good day out and enjoy the drive and each other's company then it's a good cruise in my mind.