It's hard to say exactly how many come from the national pool. As a good guess, you take the 535 MOCs and multiply by 1.2 (given that some will have 2 slots in a given year). Thats roughly 650. Add another 100 for Presidential. That's rougly 750. There are a few other sources of noms (ROTC, etc.). Given the ~1300 offers of appointment, that leaves quite a few.
However, complicating the situation are the following factors:
-- NAPS and Foundation -- they must apply for noms but, if they don't receive one, they are still generally guaranteed an appointment if they meet certain requirements
-- Some MOC slots go unfilled
And probably others that I'm forgetting.
USNA tries to notify everyone by April 15 that they: (1) have been offered an appointment, (2) are waitlisted, or (3) have been turned down. I've never seen the letters but I would guess that (2) and (3) look alike from the outside.