>expected the number of helicopters made per year to be a lot higher.
Why? Think about the number of times you (or an average person) has been on a airplane vs a helicopter. For me, the difference is approximately 2 orders of magnitude. 100,000 airplanes manufactured per year seems quite high (Boeing and Airbus make about 1000/year, each [0]). As mentioned below, aircraft last quite a long time.
>presumably military helicopters are being made in larger quantities?
Again, why? Helicopters probably reached their peak usage during the Vietnam War, when about 12,000 were used [1] over 20 years.
[0] https://leehamnews.com/2015/02/03/airbusboeing-production-ra...