An effective way to pull stories in Sprint
There should be two types of the backlog in every project. One is product backlog and another one is sprint prioritized backlog.
On day 1, the team, SM, and PO should discuss the stories together, which they think they can complete within the sprint. The sprint backlog is prepared according to sprint goal and team capacity. All stories should be completed within the sprint. However, in order to deliver a 100% sprint, it is advised that all stories of sprint backlog should not be pulled together in the active sprint. If there are 5 developers in the team, only 5 stories should be pulled from sprint backlog into the sprint, once sprint starts. Once a developer completes his/her assigned story, then only a new story should be pulled from the sprint backlog. This is the right way to pull stories in a sprint from sprint backlog.
There might be a scenario, when a developer may face any blocker or defect or may spend more time on a particular story. This is not necessary all stories of sprint backlog can be delivered but with the above practice, all stories in a sprint can be delivered.