This is responsobility of Product Owner and Development Team.
In article Roles in Disciplined Agile Delivery is written following:
Product Owner. In a system with hundreds or thousands of requirements
it is often difficult to get answers to questions regarding the
requirements. The product owner is the one individual on the team who
speaks as the “one voice of the customer”. He or she represents the
needs and desires of the stakeholder community to the agile delivery
team. As such, he or she clarifies any details regarding the solution
and is also responsible for maintaining a prioritized list of work
items that the team will implement to deliver the solution.
The key word is "prioritized". Team should take in iteration/Sprint only most prior items. And all decision about prioritization is Product Owner's responsibility.
But priority based not only by business value. In Scrum Guide is written follow:
The Product Owner is responsible for maximizing the value of the
product and the work of the Development Team.
For example: there are two stories with highest priority, A and B. Forecasted capacity for next sprint is enough only for one of them. Story A have 0.9 abstract business value point, Story B have 1.0 abstract business value point. But there is a dependency between A and B stories, and if Team will implement A first, time for B implementation will reduce twice. In that case Product Owner can make A more prior than B, to maximizing the value work of the Development Team.
That one of the reasons, why Product Owner should present on iteration/Sprint planning: datailed of story implementation may affect his decisions about prioritization.
In the other side, forecast of next Iteration/Sprint capability is Development Team's responsibility. It mean that Product Owner can't force Development Team take more tasks than they decide to take themselves.
Iteration/Sprint Planning is always compromise between what Product Owner wants and what Development Team really may do in this time period.
This is another quote (Scrum and XP from the Trenches) that sums up all that is written above:
The reason why the whole team and the product owner have to be at the
sprint planning meeting is because each story contains three variables
that are highly dependent on each other: Scope, Estimate, Importance.
Scope and importance are set by the product owner. Estimate is set by
the team. During a sprint planning meeting, these three variables are
finetuned continuously through face-to-face dialog between the team
and the product owner.