Or is that I need all my letters listed before submitting amcas, and just you can assign more letters to schools after submission as long as they were listed before submission?
if I assign 1 letter to the school now I am applying to today for my primary to get verified, I can later add a letter to amcas, and then assign it to this same school?
why does amcas say "once you assign letters to a school" it's final. Does that mean I shouldn't assign any letters yet if I feel ill add another to amcas in the future?
It means you can't change your mind and unassign them. Once you designate a school to receive a given letter, that school will get that letter when/if it shows up.