Crew Certificate permits one to be armed on board, usually paired with normal gun permit according to registration county rules.

Approximately 1/3 of US commercial pilots have had military service. Expect they’re all armed. The pilots I know aren’t ex-military, and yet are still armed.

9600 guns were purchased for the program (over three years) There are 110k commercial pilots. So, roughly 10% for the lower bound. Assuming a career of 30 years as an airline pilot, that’s 3666 new pilots per year, suggesting that every new pilot could be armed. I expect it’s much closer to 10%.