Not that anyone cares, and this is of course statistically not reliable for anything important, but I was personally curious the last 2 days around my area (subdivision) and work area (main parking garage). Granted, these are small samples, and YMMV
subdivision, cars parked in driveways or in overflow street areas :
day 1: 47 , 16 had either front, or front+rear dashcams
day 2: 39 , 15 had either front, or front+rear dashcams
13 were the same vehicle , many people play car "tetris" between the driveways, garages, and street overflow, so hard to gauge further
work parking garage (only 1 of the 7 levels)
day 1: 203 , 91 had either front, or front+rear dashcams
day 2: 205, 102* had either front, or front+rear dashcams
- denotes that possibly 102, but the front windshield was illegally tinted to such a degree I couldn't be certain if it was a dashcam or a radar detector
So very anecdotally, around 40% of people in my "orbit" have and use these devices. I would never have thought the # was that high, but then again, I never would have thought opening a thread about dashcams people would be asking why they exist and people have them. It prompted me to survey how ubiquituous they are, at least to the degree I can determine.