The maximum arity N of a function type that's implemented as a trait scala.FunctionN. Functions of higher arity are possible, but are mapped in erasure to functions taking a single parameter of type Object[]. The limit 22 is chosen for Scala2x interop. It could be something else without affecting the set of programs that can be compiled.

The maximal arity of a function that can be accessed as member of a structural type

The maximum number of elements in a tuple or product. This should be removed once we go to hlists.