First; there aren't any actual stats of multiple nominations vs appointments. At least none that I've seen. Also, certain nominations like Presidentials, are pretty much automatic nominations. On average, at least in the past, there were around 500+ presidential nominations. As long as your parent(s) are currently active duty for more than 8 years, or they are retired military, a similar point systems for guard/reserve, the applicant automatically gets a presidential nomination. The thing is however, no more than 100 appointments can be given using the presidential nomination. But in reality, just about everyone who has a presidential, is also applying for a MOC nomination. So having 2 nominations isn't really that impressive. It just gives you more than one slate to compete against.
Now, concerning MOC nominations, that a little different story. Because of the large number of applicants and the very few nominations by a MOC, having multiple nominations does impose the human factor among board members. What I mean by that is; it's only natural for a board member to wonder WHY a specific applicant would receive more than one MOC nomination. As others have said, most MOC's communicate with each other and try to maximize the number of individuals who receive a nomination and are therefor competitive. If a person receives more than one MOC nomination, there's usually only one of 2 reasons. 1) There are very few applicants in that state/district, so certain applicants might have more than one MOC nomination in order to fill a MOC's slate. 2) A particular applicant actually walks on water. So the human nature of a board member when they see an applicant with more than 1 MOC nomination is: "WHY?" If they see the individual is from a small populated state like Wyoming; and there's more nominations than applicants; it might not pull too much weight. However; they may look at the Multi-Nominated individual as the "Best of the Litter". From a large state, an individual with multiple nominations may be seen as someone who probably is much better than most/all of their state/district competitors. And the MOC's are saying that this individual really needs to be looked at more closely.
Something to remember however: If a particular applicant is THAT GOOD, and they "Walk on Water", the MOC has the option of presenting their slate of nominees with a "PRINCIPAL" nominee as the #1 slot. In which case, it doesn't matter if the academy wants this person or not. Doesn't matter if they "Prefer" a different candidate from that state/district. If that applicant who is listed as a "Principal" nominee is "QUALIFIED" to air force standards, then the academy MUST give that individual an appointment. They have no choice. And that is what I actually prefer MOCs do. If they really believe an individual is the "Best of the Best of the Best" of those applying, then instead of giving them 2-3 nominations, have 1 MOC make him/her their "Principal" nominee and save all the other nominations for others so they can compete for the other state slots, or compete in the national pool. But again, no, there are no stats that show multi-nominations vs appointments.