You can use the same letters. Some applications require certain types of teachers, while others don't. Just make sure you are following those directions and you're good! When my stepson did his, he went to the teachers he planned to use and let them know he would need several recommendation letters. He asked them to supply 4 hard copies, sealed in an envelope with their signature on the back, and continued following up with them about submitting recommendations online for USNA and NROTC (some of these emails went to their spam folders, so is good to follow up!). Same thing with transcripts. Ended up needing to open some of these letters to scan for an online application, so was good to have extra copies.