How much of the sprint backlog must be defined during the sprint planning meeting?

The answer is "Enough so the Development Team knows how to do what it has committed to do and to start the first several days of the Sprint".

The sprint backlog refers to a rundown of errands distinguished by the Scrum group to be finished during the Scrum run. During the sprint planning meeting, the group chooses some number of item excess things, for the most part as client stories, and recognizes the errands important to finish every client story.