It is said that Mont St-Michel sees 3.5 million visitors each year. That's an average of 10,000 tourists per day. Most of them will visit in the warmer weather, I imagine, so it's probably not at all unreasonable to have 25,000 visitors per day at the peak of the season. It can get quite crowded.
These people have climbed about halfway (or a little more) to the top. When they complete their climb they will be rewarded with a spectacular panorama.