I would start with asking your school counselor for information on boys/girls state (soon, date approaching) Great program via American Legion, looked after very favorably by academies, and a great experience. Our son has gained immeasurably from the experience both at Boys State and Nation and gained lifelong friends and mentors.
The fact that you are thinking ahead is a good thing. Keep doing what you are doing and look for opportunities to lead and compete where ever you can be successful. Do things you like that you are passionate about. Practice your SAT/ACT and try to keep improving. If you think you would do well, try talent contests, speech/essay contests, put yourself out there. You never know!
In terms of numbers, this is such a complex process it is hard to say. Look at prior year's class profiles at each academy and you get a pretty good idea of what they are looking for. There is a human element to this with interviews and essays so you cannot just crunch numbers. Don't go crazy over that part, advice easier said then done. I am guilty of over thinking this myself.
Keep a binder with your accomplishments in it so you can readily build a resume and complete apps. And apply to the seminars at each academy you are interested in, think the apps open first of the year for most.
Also, I am just a mom of a current candidate who has managed to get nominations, I am no expert. No appointments yet. Fingers crossed. My best advice is to be yourself, and always try to improve where you can.