Do instructors have the right to know how many students test positive for COVID-19?
In the university I work for that would make no difference: everybody physically present is tested negative, as those who are tested positive are not allowed to enter the building, and this is checked.
The bigger problem I see with giving this information to lecturers is with smaller courses. It would be very likely that anonymity can no longer be guaranteed. Moreover, there is no clear cutoff point that distinguishes between small and big courses. So I understand the universities that do not give that information.