I suspect for most of the second level domains, there's only one registrant of a third level domain.

However, I also suspect that for most of the third level domains, they're on a second level domain with multiple registrants. And several of those shared second level domains will have a lot of registrants. If the registrants are in different jurisdictions, it becomes a pretty nasty process to develop international consensus, vs just keeping the status quo going (which IMHO, can't be that hard).

Still, it makes sense to have a process to normalize the single registrant second level domains, because less work.

22,000 third levels total, spread across about 12,000 second levels, about 10,000 of which have only one third level. Of the approximately 2,000 with multiples, many have all of the third level registrants being apparently close relatives.

That leaves quite a few with lots of unrelated registrants, mostly in Japan (tanaka and suzuki being the biggest).