An essential parameter of physical, propagation based MIMO channel models is the number of multipath clusters. In this paper we determine the total number of clusters and the number of dominant clusters in the angle-of-arrival/angle-of-departure domain based on comprehensive indoor MIMO measurements at 5.2 GHz in a cluttered office environment. For the dominant multipath clusters we find a mean number of 7.6 with a standard deviation of 2.4, whereas the total number of clusters varies strongly even within a single room of the environment.