There is no general rule - as the minimal amount depends on the system, its stakeholders, criticality, complexity, size and/or risks.

We suggest to always explain, document or specify the following aspects:

• 3-5 central quality requirements, expressed in scenarios.
• Context view and external interfaces
• Brief explanation of your solution strategy, the most important decisions influencing the implementation or operation of the system.
• Building block view, level 1.
• Most important crosscutting concepts.