There is no Early Action for USNA. There are some Letters of Assurance, but they still require certain things -- they are not offers of appointment in and of themselves.
USNA doesn't publish the number of LOAs. However, they have said they are used somewhat sparingly because they constrain USNA's ability to form the class if really great candidates show up later in the cycle.
Numbers for USNA are in the class profiles -- USNA does one for each class. I think it's on the usna.edu website, probably under Admissions. Tells you the # of applications (realize that many don't complete their apps), number of candidates 3 Q'ed, number with noms, number of appts offered, and number accepted.