A Scrum Team is intentionally small and is composed of exactly three accountabilities: one Product Owner, one Scrum Master, and the Developers who do the work of delivering a valuable Increment. The optimal size is ten or fewer people, which is small enough to remain nimble while being large enough to complete meaningful work within a Sprint. The team is designed to be both self-managing and cross-functional. A self-managing team internally decides who does the work, how it gets done, and how to turn the Product Backlog into Increments. A cross-functional team possesses all the skills needed to deliver a valuable Increment without depending on people outside the team, which reduces handoffs and waiting time.
The Product Owner's primary responsibility is to maximize the value of the product resulting from the work of the Scrum Team. To be effective, one Product Owner must be assigned to one Scrum Team, although they may represent the interests of many stakeholders. The Scrum Master's primary accountability is to establish Scrum as defined in the Scrum Guide, remove impediments, and coach both the team and the organization. Within a single Scrum Team, the Product Owner and Scrum Master are separate accountabilities and cannot be combined into one person, and the Scrum Master also cannot simultaneously serve as a Developer. These boundaries preserve clarity of purpose and the system of checks and balances that allows empirical decision-making to flourish.