The blind interception process of multiple-input multiple-output (MIMO) signals have recently gained more attention. The number of transmitting antennas is an important information for many blind MIMO algorithms. In this paper, several algorithms for blind detection of the number of transmitting antennas are presented. These different algorithms are evaluated and a performance comparison is presented. Spatial correlation is a crucial factor for practical MIMO systems. This paper addresses for the first time the problem of blind detection of the number of transmitting antennas in spatially correlated MIMO systems.