4+ million points is working well in BTB. Detail level is high along the aerial scan lines and there's less than a meter between scan lines, so it's probably in the intermediate density. I classified the points in BTB and am gradually making progress... it's a historic track that was turned into a golf course, so sussing out the original circuit's route is, uh, fiddly because there's no obvious pavement. I'm doing it in point-to-point segments as a guide for the final circuit. The terrain will look fabulous!
Another historic track has a sparse density aerial scan. Because most of the track still exists, the points have been easier to classify, just wish the scan were a little higher density.
Went to look for LIDAR data for my home track, Portland International Raceway, and discovered our government's most recent scan contains 34 million points... hmm. Don't think I'm up to working with that much data just yet, but it is available if someone is looking for a project!