A 20×200 mini storage building will be able to provide you with 4,000 sqft of space, which could be used to create ten separate units at 20×20 each.
The benefits to having a prefabricated kit are that you can have your mini storage building constructed in a matter of a few days or a few hours depending on the size of the kit and your experience.
This is because the parts that make up your building is made elsewhere, then brought to your location for assembly.
Steel has many advantages, as it is both robust and resistant to the weather.