Elastomeric materials are available in a wide variety of hardnesses, from 20 Shore A to 90 Shore A for thermoset rubbers, to even harder materials (Shore D scale) for thermoplastic elastomers.
The most common hardness range for materials is from 50 Shore A to 80 Shore A, with most sealing products being made from materials with a hardness of 70 Shore A. The actual hardness which will be selected depends upon your exact application.
There are some restrictions on the use of very hard and very soft materials in terms of manufacturing limitations. Parts with complex geometry or deep undercuts can be difficult to manufacture from very soft (< 30 Shore A) or very hard (> 80 Shore A) materials.