The fundamental unit of Scrum is a small team of people, a Scrum Team. The Scrum Team consists of a Scrum Master, a Product Owner, and the Developers. - Scrum Guide
The Scrum Team functions as the center of the creation of Increments to achieve the Product Goal. As the "fundamental unit of Scrum,” Scrum happens within the team, and therefore, regardless of your role, you must take care of the team as the foundation from which everything else is based.
Within the Scrum Team, there are three roles:
- Product Owner
- Scrum Master
If someone has the role of Scrum Master or the role of Product Owner, they could also have Developer’s role, although it is not required. No one should be the Product Owner and the Scrum Master simultaneously since both roles have opposing interests.
Scrum does not recognize any hierarchy within the team; there are no sub-teams or more roles other than those mentioned previously.
Since Scrum is designed to respond inexpensively to the changes inherent in complex environments, both the structure and size of your team should match this context.
A Scrum Team typically has no more than 10 people. Having a larger team has proven not to be efficient enough to change direction and make decisions quickly. A Scrum Team should not be small either; it should have the right size so that the Increment produced in each Sprint has considerable value.
Large teams are divided into separate Scrum Teams of no more than 10 people who work on the same product and share the same Product Owner and focus on a single Product Objective and Product Backlog. If you wish to learn more about the use of Scrum on a large scale, I recommend researching a framework derived from Scrum called LeSS (Large Scale Scrum).
An essential factor to consider related to team size is that within a Scrum Team, all the necessary skills to deliver Value Increments at the end of each Sprint are expected to be present. This implies that the Scrum Team does not need the intervention of people from outside the team. We call this feature cross-functionality. A Scrum Team is a cross-functional team.
No one outside the team intervenes, deciding what work the team performs, when it does it or how it does it. Decisions about these three aspects are the sole responsibility of the Scrum Team. The Scrum Team decides the what, when, and how. We call this feature self-management. The Scrum Team is entirely self-managed.
Due to its cross-functionality and self-management capacity, the Scrum Team is responsible for doing whatever it takes to deliver a value Increment at the end of each Sprint. This includes the construction of the Increment and includes collaboration with stakeholders, research, testing, negotiation, training, learning, etc.