Honestly, as long as one member of your party shows up, you are fine (not counting anything that is prepaid like dining packages, etc.). As for more people than on the reservation, it really depends on the situation at the restaurant. For example, if they are close to hitting maximum occupancy, they may not be able to accommodate a 4th person at a reservation for 3, even though the table could handle 4 people. So it is always best to get the maximum number that will go, but don't intentionally go higher than you would expect to have either.
Also, when we couldn't increase the party size, we have had success in making a second reservation for the same time as the first reservation (even day of) and asking to be seated either at the same table or at least near each other.