Hopefully you have been able to attend practice rounds at a studio so you can experience what it's like to dance on a relatively crowded floor without getting rattled or intimidated. I assume at your age that stamina shouldn't be much of an issue. If it is a straight final, which I highly doubt based on the entries I've seen for MAC in the past, you'll be dancing CSRPJ (or whatever the legal sequence is nowadays) all in a row. If it's, say, a quarter final, they'll likely split it into 2-3 groups equal in number, you'll do one dance, wait in a designated corner or area until the next group does the same dance, and then repeat the process until all couples have completed all dances for the event. Depending on how the other events of the day are interleaved, you may have quite a bit of down time until the next round. You also may end up with back-to-back prechamp/champ events, which can be brutal. Most comps are very good about announcing what's coming up soon and/or having a video display of which couples were recalled for the next round. Keep an eye on the schedule and always know where your partner is. It's very embarrassing to hold up the competition because someone is missing. Often if you aren't on deck and ready to dance when you're called, tough luck. You're done.
Check youtube for MAC videos of these events so you can see what it really looks like. I'm sure there must be a lot of them.
Is that what you were wondering about, or did you have other specific concerns?