I don't have any particular interest here, as I'm quite ambivalent about guns but the 600k-2.5m is hotly debated and comes from phone surveys. The CDC cites Priorities for Research to Reduce the Threat of Firearm-Related Violence (2013)[1] when reporting that number. The source says it may actually be as low as 100k uses per year.
Funny thing, the first study was done by a gun control group, and long before shall issue concealed carry made self-defense a lot more common by allowing it outside the house. Their number, which did not include a person using a gun more than once a year: about a million.
But it doesn't matter, it's an unalienable right enshrined in the Constitution, "and if it saves just one life...."
[1] https://www.nap.edu/read/18319/chapter/3?term=defensive#15