There are 9 barracks which includes the newest one, Davis. There are 4 regiments (1-4), with 9 companies (A-I) per regiment. Each academic company (ie A4, D1, B2) is in the same barracks, and multiple companies within each regiment may be in the same barracks, or different ones.
Your new cadet will be in a Beast company, and then scramble to an Academic company during Reorgy. Their PO box # has nothing to do with the barracks, but they will keep the same PO Box # all four years. If you go to A-Day, Plebe Parent Weekend, Parent Weekend or Graduation, you can go inside their barracks.
Each cadet class scrambles at some point, usually from Beast to Plebe year, and then again sometime during their 4 years. Class of 2018 scrambled after Beast, again after Plebe year (or was it Yuk?) and again after Cow year. I've lost track. My Firstie will be in his 3rd or 4th barracks starting in August.