In 2016, Politico reported that the total number of trombone, trumpet, keyboard and other instrument players [in the U.S. military] stands at about 6,500.
That’s a lot of Souza marches, but the State Department fields a bigger squad of diplomats. There are 8,106 Foreign Service officers, according to a State Department report. (The State Department has about another 5,700 people to support the diplomats, but they don’t do direct diplomatic work.) Still, there are a good 1,600 more diplomats than musicians.
For what it is worth, my (late) father was in a military band when he served in the 1950s.