The Developers are the ones who determine how many Product Backlog items they select for a Sprint. The Developers are self-managing and decide how much work they can do in a Sprint1. The Product Owner and the Developers collaborate on the scope of the Sprint during Sprint Planning, but the final decision is up to the Developers2. The Product Owner, the Scrum Master, and the stakeholders do not have the authority to tell the Developers how many Product Backlog items they should select, as this would violate the principle of self-management[3][3].
References:
1: The Scrum Guide, November 2020, p. 6
2: The Scrum Guide, November 2020, p. 10
[3][3]: Understanding and Applying the Scrum Framework, Scrum.org, accessed on December 16, 2023