The amount of the Sprint Backlog that must be defined during the Sprint Planning event is enough so the Developers can create their forecast of what work they can do. This is because:
Sprint Planning is an event where the Scrum Team plans for the upcoming Sprint. The purpose of Sprint Planning is to align the entire Scrum Team around a common goal and a plan for delivering an Increment that meets that goal.
The Developers are accountable for creating a Sprint forecast, which is a selection of Product Backlog items that they intend to work on during the Sprint. The Sprint forecast should be realistic, achievable, and valuable.
The Developers are also accountable for creating a plan for how they will deliver the selected Product Backlog items as a “Done” Increment. The plan may include tasks, dependencies, risks, estimates, or other information that helps them organize and manage their work.
The amount of the Sprint Backlog that must be defined during Sprint Planning may vary depending on the context, complexity, and uncertainty of the product development. However, it should be enough so that the Developers can create their forecast of what work they can do and have a clear direction for the first few days of the Sprint.
Other options, such as just enough to understand design and architectural implications, the entire Sprint Backlog being identified and estimated by the end of Sprint Planning, or just enough tasks for the Scrum Master to be confident in the Developers’ understanding of the Sprint, are not valid answers as they do not reflect what must be defined during Sprint Planning or what is required for creating a Sprint forecast.
References:
[Scrum Guide], page 14, section “Sprint Planning”
[Scrum Guide], page 7, section “Developers”
[Scrum Guide], page 15, section “Sprint Backlog”
[Scrum Guide], page 14, section “Sprint Planning”
