One of the major problems in clustering is the need of specifying the optimal number of clusters in some clustering algorithms. Some block clustering algorithms suffer from the same limitation that the number of clusters needs to be specified by a human user. This problem has been subject of wide research. Numerous indices were proposed in order to find reasonable number of clusters. In this paper, we aim to extend the use of these indices to block clustering algorithms. Therefore, an examination of some indices for determining the number of clusters in CROKI2 algorithm is conducted on synthetic data sets. The purpose of the paper is to test the performance and ability of some indices to detect the proper number of clusters on rows and columns partitions obtained by a block clustering algorithm.