We get the prioritized list, we review our other commitments in the upcoming Sprint, and starting from the top of the list we go down and developers pick which ones that they will work on. Since we've groomed these stories, we have an idea of complexity and one or more developer may have already taken the time to identify exactly what needs to be done.

We don't commit before hand to deliver a specific number of stories, or a specific number of story points. That would never work. Rather we focus on what individuals believe they can achieve. Our velocity has become pretty consistent and our delivery of story points is close to 100%