Whilst mu metal is available in many different gauges and indeed a single layer of mu metal is suitable for most commercial applications, optimal shielding results are obtained when a shield is constructed of multiple layers of mu metal.
In theory each layer should be of a light gauge and the shield constructed of many layers [...]

What is field attenuation?
This is also known as the shielding factor (S) and is a ratio of the magnetic field strength outside of the magnetic shield (Ha) and the resultant field on the inside of the shield ie Ha/Hi (no units) or S = 20 x log(Ha/Hi) (Db). There are various formula based on the [...]

Mu Metal Mechanical properties at ambient temperature (reference guide)
Density: 8.7 [g/cm^3]
Youngs Modulus: 225 [GPa]
Poissons Ratio: 0.29