Hi Alex,
Manual trackers will be used with a higher priority if you run auto as well. Which means, if they are not better than the auto tracker, you might ruin the shot.
You might know most of this from the old days, but to make sure I mention it and based that others read along as well.
The minimal amount of tracker will be around nine, some might say lower, some higher. The fact is, they need to be uninterrupted and very precise. Avoid anything in the distance, avoid reflections, and other things that might change based on movement (camera, scene, etc.).
You should use a lens calibration first, as any distortion makes part of the picture move slower than faster. This will not sit well with just a few trackers.
I typically start in the middle, place a lot of AutoTracker (Create Auto Tracks) and track, then move the time slider to each end and do it again. Based on the length of the track, I do it more or less often. It takes less and less time, the more you do it.
After that, I go through the whole clip and delete typically tracker that seemed to have fetched a target that is not really one.
Motion blur, Rolling Shutter, and lock-off shots (tripod) might cause problems. Whereby I had good results with pan shots, and locating the camera with the camera Calibrator.
Please keep in mind that the Lens length is based on a full-frame, but RED, for example, can use less space of the sensor than its maximum size.
With the right Lensprofile, a cleaned-up 2D track, and lens length entry, things should generally work.
More details:
https://www.cineversity.com/vidplaylist/motion_tracking_object_tracking_inside_cinema_4d/motion_tracking_object_tracking_inside_cinema_4d_introduction
My best wishes
