The Product Owner controls the order of the items in the Product Backlog, but not how many are done each Sprint. Instead, the team decides how many to do. This decision is made in Sprint Planning and, of course, should be made in collaboration with the Product Owner, but ultimately the Product Owner must let the team freely decide how many items are planned. This freedom allows the team to be truly motivated and committed to the work as well as being collectively accountable for getting that work done. If the Product Owner forces a team to take on more work than it feels is within its capacity, then that dis-empowers the team, moves accountability for getting work done to the Product Owner, and completely dis-ables the team from getting to a high-performance state.
To learn more about Product Owners, visit the Scrum Team Assessment.