We can use the count property to determine the number of items in a set. Here is an example of how to use this method:

var mySet = Set<String>()  
mySet.insert("One")  
mySet.insert("Two")  
mySet.insert("Three")  
print("(mySet.count) items")

When executed, this code will print the message 3 items to the console because the set contains three items.