Well you see
Kamen rider Decade insert one card into his belt to transform.
Kamen rider Double insert 2 Gaia memories into his belt to transform.
Kamen rider Oz insert 3 core medals into his belt to transform.
Kamen rider Fourze insert 4 switches into his belt to transform.
Kamen rider Wizard even thou he wear one ring he has to put all five fingers in his belt to transform.
Nice! That is why Toei rocks because their simple creative ness works!