This paper deals with a new challenge for cognitive opportunistic receivers : base station number of antennas estimation. This knowledge allows to get a better understanding of the Signal to Noise Ratio and of the achievable bit-rate with the BS. This task is achieved by taking benefits of the orthogonality between the pilots patterns used by the base station antennas and making use of a the Pilot Induced Cyclo-stationarity detector proposed by Sochelau et al. To the best of our knowledge our proposed algorithm is the first technique that can estimate any number of antennas using only one antenna at reception and without any knowledge of the pilot sequence.