I'm tracking a shot where none of the markers stay in frame the whole time, so I make up for the markers lost by adding new ones. I make sure there's at least 9 active markers every frame, but it still tells me I need 8 common tracks on each keyframe. How can I figure out which parts of my shot don't have enough markers?
You don't need the markers to stay in the frame for the duration of the shot, but you do need to have at least 8 successful trackers for a section from which blender can solve the scene.
The range of frames used for solving can be set in the solve section Keyframe A and B
If you can't decide which range of frames is best, you can let Blender decide for you by enabling the Keyframe option on the solve tab.
Please read through the following link: How can I get better results when doing camera motion tracking?