You don't say which airplane you use but, from you saying it always boards 20 passengers, it seems you are using FS Labs, because that's a value they initially set for the passengers number by default, before any actual payload is configured.
When GSX is under control of the airplane, which has set the passenger number on its own, the option to ask how many passengers you have is not displayed, since it's supposed the airplane already set it correctly, this is the correct behavior.