Cognitive Radio has become an effective theory to solve inefficient spectrum usage. The traffic of Wi-Fi data increases rapidly because of smart phones and netbooks. IEEE 802.22 standard supposes a cognitive radio system which consists of one licensed primary user (PU) and multiple unlicensed secondary user(SU)s. The SUs utilize the licensed spectrum while the PU is absence. In this paper, we consider to reuse the fallow TV (PU) spectrum for Wi-Fi (SU) data transmission without causing any harmful interference to incumbents. The optimal threshold number of SUs to detect the presence of PU in the constraint of minimizing detection error probability is derived. As a result it increases the capacity of traffic in the Wi-Fi zone.