Mobile data offloading has been emerged as a promising solution to manage high data traffic imposed by mobile users. This technique exploits the frequency spectrum of some overlapping networks such as WiFi in cellular networks. Deployment of minimum WiFi access points (APs) is the main challenge in mobile data offloading which leads to the low cost of CAPEX and OPEX. In this letter, we investigate the impact of a relay-based cooperative MAC protocol on the spectrum efficiency of mobile data offloading and consequently on the WiFi APs deployment. Two algorithms are presented to compute the throughput gain and the minimum number of WiFi APs in two modes of cooperative and non-cooperative MAC protocols. The analytical results indicate that the cooperative MAC protocol can provide a reduction of up to 44% in the number of WiFi APs in comparison to non-cooperative mode.