In complex domains, you are only able to know the outcomes in hindsight. Empiricism, from which Scrum was founded on, asserts that knowledge comes from experience and making decisions based on what is known. New knowledge will emerge as work is being done.

During Sprint Planning the number of items selected from the Product Backlog for the Sprint is solely up to the Development Team regardless of the number of Development Teams working from the same Product Backlog. Only the Development Team can assess what it can accomplish over the upcoming Sprint.

Explanation
Explanation:
Product Backlog items should be clear enough and have enough information for the Product Owner and Development to understand the work that needs to be done and to create a forecast of Product Backlog items to implement the Sprint Goal. Nothing stops the flow of Sprints, for example, we do not delay the Sprint because the items are not ready. In such cases, the "unready" items would be selected for the Sprint anyway and refined during the Sprint.
In the case that the Product Backlog items are not clear enough, the Development Team forecasts the most likely Product Backlog items to meet the goal and create a Sprint Backlog based on a likely initial design and plan. Once the time-box for the Sprint Planning meeting is over, start the Sprint and continue to analyze, decompose, and create additional functionality during the Sprint. At the end of the Sprint, the Scrum Team can discuss in the upcoming Sprint Retrospective why this happened and what changes will make it less likely to recur.

The Scrum Framework only recognizes three roles although others may be needed to help the team build the most valuable product possible.

The Scrum Master serves the organization in several ways, including: Leading and coaching the organization in its Scrum adoption; Planning Scrum implementations within the organization; Helping employees and stakeholders understand and enact Scrum and empirical product development.