Creates and returns a mutable set with a given initial capacity.
- iOS 2.0+
- macOS 10.0+
- Mac Catalyst 13.0+
- tvOS 9.0+
- watchOS 2.0+
The initial capacity of the new set.
A mutable set with initial capacity to hold
Mutable sets allocate additional memory as needed, so
num simply establishes the object’s initial capacity.