Just for the sake of doing some math,
Correps.com lists 1,237 junior corps (since about 1925)
If we assume that each corps averaged 55 members (arbitrary, I know that historically 128 members was only the top 20 or so corps since the mid 1960s) , then there are 68,035 corps alumni out there. Now, also figure that say, 25% have passed away (also an arbitrary guess) - that leaves 51,026 alumni. Not the 2 million kids in bands, but still a core group of fans.