I don't know how many but I can tell you I have seen cut up Garands since my early days. Rock Island, Letterkenny, and Red River are just three facilities that sold off "scrap" piles consisting of cut up Garands, that I know of.
The piles at Rock Island were of the size of gondola cars, the Letterkenny piles were open head 55 gallon drums, and I am not sure about Red River, did not see it only heard about it.
A few of the larger dealers of today got their start purchasing cut up Garands and 03's by the ton, breaking the stuff down, refinishing the components and selling them off.
Earliest time period I am familiar with is around 1974-75.