How many elastic constants of a linear, elastic, isotropic material will be?

Isotropic materials have the same properties in all directions. The number of independent elastic constants for such materials is 2. out of E, G, K, and ?, if any two constants are known for any linear elastic and isotropic material than rest two can be derived. Examples are steel, aluminium, copper, gold.
Orthotropic materials refer to layered structure such as wood or plywood. The number of independent elastic constants for such materials is 9.
Non isotropic or anisotropic materials have different properties in different directions. They show non- homogeneous behaviour. The number of elastic constants is 21.