What is the minimum number of objects needed to form a retention cycle in memory?

The minimum number of objects required to form a retain cycle in memory is two. This occurs when two objects hold strong references to each other, preventing the memory management system from freeing them even if they are no longer needed.

Example in Swift:

class A {
    var b: B?
}

class B {
    var a: A?
}

let objectA = A()
let objectB = B()

objectA.b = objectB
objectB.a = objectA

In this example, objectA and objectB hold strong references to each other, creating a retain cycle.