Every MOC will be different, some will do just the 10 (basically perfunctory), some will do more. I know for DS they had @20 for 10 slots. Remember, that although you personally, may only be asking for the AFA others are requesting noms for multiple SAs. Thus, you may have AFA as your number 1 and USNA as number 2, with USMA as number 3, and they give you a nom only to USMA. So don't read into how many people are there. Especially, since highly competitive areas have more than 1 date and time for interviews.. They could have them spread across several weekends, or in very large states have several locations for interviews...if you think about it, it is unrealistic that the candidate from San Fran make it to LA. Plus, there is the ones you can easily forget about...the military dependents that their parents are not assigned in that state. Typically, they do phone interviews with them.
My point is don't put to much into getting there and counting the other kids in the room, if 6 are there for the AFA, maybe 6 more are going the next week, or maybe 6 more did a telephone interview.
BSA, I know from our close friend who has a DS at USNA, and lives right outside of Denver, they were informed of his appointment before Thanksgiving, thus the turn around time at least 2 yrs ago was very quick from interview to apptmt. Side note, his #1 choice was USNA, he got the nom for the AFA. The girl who got the USNA nom had the AFA as her #1 and they called the Sen to have it flipped. She is at AFA.