You definitely can use the same one for all your MOC's (senators and rep's).
The academies have different requirements (not sure if you're applying to more than WP). I know for both AF and Navy, you had to have your English and your Math teacher (from 11th or 12th grade) and they had to be submitted online, so those would have to be different (or at least YOU wouldn't be seeing them at all). AF also required a third letter from another teacher of your choice - online again. But also allowed 3 additional letters to be faxed or mailed - from anyone. My son used his soccer coach, principal, and class sponsor (for student government). But Navy didn't ask for/allow any additionals. These may be different from what you want for a MOC if you want to have a better representation - ie one teacher, one sports type, and one leadership type - just to show you're well rounded.
I will strongly recommend that you talk to your teachers before school gets out this year and get a number that you can contact them at and/or a home email so you can get a jump on stuff over the summer. My son had done this with most of his contacts, but listed his AP Chem teacher as his third teacher for AF. Turned out she was pregnant and moving over the summer and he spent forever playing phone tag with the office (who wouldn't give out her contact info) and trying to get her to call him so he could get the right email to AF and she could actually reply to it! Any that you think you can get typed (bosses, coaches, pastor, principal - anyone who won't have to be on the "online" list), you could even ask for before the summer and it would make life easier!
Also be sure to find out your MOC's requirements first - I know some on the boards required them sent directly, sealed, to them. Ours required my son to collect all 3 and mail them together as part of all of his application packet - they wouldn't accept them separately.
Good luck!