In the context of Scrum, the development team is responsible for turning a backlog of items into a potentially releasable increment.
This potential part is very important because in the end, releasing or not releasing is a business decision.
For the development team to produce their best work, every backlog item they work on must be independent, testable and able to be DONE by the development team without the need of someone else.
This means that good teams are focused on features, not technologies. They are cross-functional and contain all the knowledge necessary to do what must be DONE.
The perfect team size is always a debate. In this discussion, I always like to fall back on the scrum guide for guidance.
Optimal Development Team size is small enough to remain nimble and large enough to complete significant work within a Sprint.scrumguides.org
No fewer than 3, or having more than 9. This excludes both the Scrum Master and the Product Owner.