Here is whatI learned about LORs. Every school will provide this information on their website (not being a wise guy, fyi), and pay attention to that before submitting your AMCAS application to that school because at the point of submission, you will be asked to check the boxes for the LORs you want going to that school. And for the letters, I had my writers submit them to Interfolio and then they can all be sent to AMCAS from Interfolio. Also, if you apply to D.O. schools, they will also accept those same Interfolio letters, but the D.O. process requires you to submit the letters directly from Interfolio to each particular school, not through a central clearing house.
I think UNC only asked for 2, but I don't remember for sure. And there are no LORs sent with secondaries, they are all sent through AMCAS with your primary. You can add LORs after your original submission but I would avoid it if possible because I think sometimes that could delay your APP or at the very least, make the admissions office have to do more organizing after the fact. That last sentence is not based in fact, only my thoughts, btw.