I am not by any means the expert on this, but technically I don't think you can change the # of people on an ADR, you would have to cancel and make new one, which is risky. But I have shown up for an ADR with a couple of people missing (SIL's boss called her back to work, couldn't be helped) and it wasn't a problem - that was at Chef Mickey's.
I am worried about the exact same thing for our trip next summer though. Myself, DH, and DD probably wouldn't miss an ADR unless it was a life or death emergency, but the other 6 people going with us seem a little less than committed to having such a strict schedule, and I am afraid they will end up bailing on at least a couple of ADRs at the last minute. I may make some of ours as two separate ADRs (a party of 3 and a party of 6) so that if they change their mind, I at least won't have to worry about our ressie getting messed up. If you don't mind not always sitting together, this might be an option.