The Welsh Government publishes regular updates on positive cases of COVID-19 reported to higher education institutions in Wales, which you can access here. Note that these are presented only as 7-day rolling averages and not daily reported cases. Cardiff University publishes its own data on reported positive cases among staff and students and number of students isolating here. According to these data, there have been 42 reported positive cases among staff since October.
While the numbers of reported positive COVID-19 cases in Cardiff University (and HE institutions in general) have remained relatively low, it is clear by the peak in October that the number of cases can increase very quickly, that the University screening service detects a significant and worrying number of positive cases who are asymptomatic, and that these asymptomatic tests are being carried out on a very small proportion of the University community, particularly considering that staff working on campus should be going through regular testing (at least weekly).
Even with most students still not back on campus, since the 5th January there is an average of over 90 students a day reporting to be in self-isolation, with an average of 35 a day reporting a positive COVID-19 test.