Other system landscape configurations are also possible - for example, a configuration with four systems - a development system, a test system, a UAT System, and a production system may be required. Another possibility is to have a system landscape consisting of a development system, a test system, a training System, and a production system.
However, at a minimum every system landscape configuration should have at least one development system and at least one test system, both separate from the production system. It's also recommended that the number of systems in a system landscape is not allowed to grow too large. This is due to the difficulties that may arise in managing the transport paths between the systems, and the associated problems that may arise with regard to system consistency.