If you want to know what the attendance at a racetrack is, you have two options, the official channel, and the unofficial one. If you want the official tally, you have to wait until Sunday, when the circuit, together with Dorna, publish the number of spectators over the three days of the track. Those numbers are based on ticket sales, though how precisely they are reflect the numbers at the track is a frequent topic of speculation.
If you want a more accurate assessment of how busy a track is, then the best thing to do is to canvas a few of the regular photographers who shoot MotoGP. They spend all day wandering around the track, seeing most of the grandstands and hillsides which overlook the circuit. A trained eye for detail and an excellent memory are key assets for a professional photographer, so they generally have a pretty good idea of how many people are at the track. Their estimates are usually much more accurate than the official numbers, and can differ by a surprisingly large amount from them.
So when several photographers report that the hillsides at Mugello seem emptier again this year, then it would appear that MotoGP has a problem. And given the nature of MotoGP's audience in recent years, that problem has one major cause.