Fermi energy (density of electron)

[royayash]

To find the Fermi energy I need to find the electron density of the material and that I can do by finding the density of atoms in the material in some instances this same density.
From what I gather it's not always that way Sometimes you have to multiply a certain number
For example, aluminum should double in 3 the density of atoms to know the density of electrons.
I want to know how I know when I need to double? How much I need to double?
Thank you

 

[Bill_K]

Isn't it because the valence of Aluminum is two? It's the number of conduction electrons per atom.