I believe that the fediverse.observer site can list any Fediverse instance type by number of users (though not active users).
checks
Oh, they do do active users.
https://peertube.fediverse.observer/list
Looks like the top one is phijkchu.com, at 8074 active users.
EDIT: There’s also fedidb.com:
Choose “PeerTube” as server type, and they’ll give you some data on instances too.
EDIT2: Note that another way to explore PeerTube, which may be to your taste, is that Google Video indexes PeerTube servers, though I don’t know of a way to restrict it to only PeerTube servers aside from using something like site:phijkchu.com
to restrict the search on an instance-by-instance basis. But if you search and it’s on PeerTube, and Google has indexed it, it should come up there.
Kagi also indexes videos, and lets lets one restrict the search by source of videos, with “PeerTube” being one.
EDIT3: Adding “peertube” as a search term on Google Video isn’t ideal, but it did result in videos on PeerTube hosts at the top, so maybe that could be kind of an ad-hoc way of searching on Google Video.
EDIT4: libera.site doesn’t appear to provide sortability, but it does list a video count per instance, as well as a bunch of other graphed data. Never seen it before now, though.
Have you tried using a vacuum cleaner?