How many people assume the product owner role in the scrum framework?

only one person should assume product owner role

Explanation:

A scrum product owner is responsible for a Scrum team or more than one scrum team. the product owner handles the business part of the scrum network and liaises between the scrum team and the stakeholders of  the team. the team ( engineers ) does the technical work .

The Scrum product owner manages the risk that comes with the business/work been done by the team, he also manages complex situations using the empirical tools provided.

Scrum is a framework ( agile ) for managing/solving complex knowledge work