China Neodymium And Ferrite Magnets Manufacturer & Supplier 
In magnetic shielding applications, soft iron is one of the commonly used materials, it can effectively attenuate or guide the magnetic field, so as to achieve the effect of shielding magnetic, in which the thickness is more critical, so how thick soft iron can block the magnetism?
Soft iron has a high permeability (permeability μ is much higher than air), which attracts and guides magnetic lines of force, reducing the magnetic field strength in the target area. The shielding effectiveness of soft iron depends on the material properties, thickness, magnetic field strength, and shielding design.
Neodymium magnet isolated magnetism packaging
In low strength magnetic fields, 1-2 mm thick soft iron may provide some shielding, for medium strength magnetic fields (such as those generated by ordinary NdFeB magnets), 3-5 mm thick soft iron is a more suitable choice. Soft iron of this thickness can significantly reduce the strength of the magnetic field in the target area without easily saturating it.
Magnetic permeability (μ) is a key parameter in determining the performance of magnetic shielding. Soft iron with high permeability can shield stronger magnetic fields at thinner thicknesses. For example, pure iron or high permeability alloys (e.g., pozzolanic alloys) are better suited for shielding magnetic fields than ordinary soft iron.
Note: If a single layer of soft iron cannot meet the demand, a multi-layer shielding design can be used, leaving a certain gap between each layer to avoid magnetic saturation of a single layer.
Enclosed shielding structures (e.g. cartridge, box) are more effective than flat shielding because they direct the magnetic lines of force more efficiently.
This is some information about how thick soft iron can block magnetism, I hope it will help you!
Highly relevant article;