Mobile applications (apps) are being released rapidly with the development of smart devices. Usability is a critical success factor and it is essential to conduct usability tests before launching mobile apps. The aim of this research was to find the optimal number of participants for the usability testing of mobile apps. This research involved conducting 4 rounds of usability tests using representatives of the users of a mobile app “JB-Career-Connect”, which provides a platform for establishing direct connection between graduating students and employers. Each of the testing rounds had different number of participants. Our test results show that 2 testers detected 16% of the usability issues; 5 testers detected 36% of the usability issues; while 9 and 12 testers each detected 64% of the usability issues. 9 testers appear to be more cost-effective since they performed well in other usability metrics. Our research provides evidence that 9 testers is the optimal number of participants required for testing the usability of mobile apps.