I feel like there should be a maximum to the amount of flights, because the game is ultimately centered around commercial aviation and it should push you towards that somehow. That doesn’t however mean that 20 is the correct maximum, I would personally scale it up towards 40 or so but that is up to the developers to decide.
I disagree that the game has to be centered around commercial flight. I mean, obviously it is, and that’s great, but I don’t see any reason from either a developer-resources or gameplay perspective for the restriction of not allowing a GA-only airport of any real size (beyond what the ATCT can handle). From a dev perspective GA is basically “done” as far as I can see (other than getting it to adhere to ATCT possibly).