If I remember well, it was estimated that the average pilot hit about with 2 % of its rounds. This theorically means quite a lot of rounds fired to score thoses 20 hits.
Most shots miss. The actual average hit amount is possibly less. I have seen basic heavy AA estimates around 10,000:1. However, I think the subject here is heavy Bomber vulnerability
in the real operational environment Vs Policy. The Flying Fortress was never a fortress, it suffered terrible losses, as did most bombers in WW2.
Eng