the Army has 399 bands!?!
I am not an expert on Army musical unit history by any stretch of the imagination.
However, the Army was at one point far larger than it is today — including Army Reserve and National Guard and former Army Air Force units that transferred to the Air Force after World War II. Lots of Army units have high numbers that reflect the situation back when the unit was organized during World War II, World War I, or sometimes even earlier. Just because there is a 399th Army Band today doesn’t mean there are still 398 other bands in existence.
Here is a unit page for the 399th Army Band, which includes a history dating back to World War II:
And here is an unofficial listing of military bands that seems to indicate the United States Army has fewer than 399 bands today:
For any further details, I need to defer to Army personnel who can give a better answer.