It's for optimization, at run-time you could handle functions with more arguments, but with this it can be compiled to directly call the exact function based on

It's for optimization, at run-time you could handle functions with more arguments, but with this it can be compiled to directly call the exact function based on the number of arguments statically used at the call-site and their type. To do this though, you need a defined static type for the type of Functions each of a different number of arguments. 22 is just where they stopped to keep defining those types.