This study examines the large-scale factors that govern global tropical cyclone (TC) formation and an upper bound on the annual number of TCs. Using idealized simulations for an aqua-planet tropical channel, it is shown that the tropical atmosphere has a maximum capacity in generating TCs, even under ideal environmental conditions. Regardless of how favorable the tropical environment is, the total number of TCs generated in the tropical channel possesses a consistent cap across experiments. Analyses of daily TC genesis events reveal further that global TC formation is intermittent throughout the year in a series of episodes at a 2-week frequency, with a cap of 8-10 genesis events per episode. Examination of different large-scale environmental factors shows that 600-hPa moisture content, 850-hPa absolute vorticity, and vertical wind shear are the most critical factors for this global episodic TC formation. Specifically, both the 850 hPa absolute vorticity and the 600 hPa moisture are relatively higher at the onset of TC formation episodes. Once TCs form and move to poleward, the total moisture content and the absolute vorticity in the main genesis region subside, thus reducing large-scale instability and producing an unfavorable environment for TCs to form. It takes $\sim$2 weeks for the tropical atmosphere to remoisten and rebuild the large-scale instability associated with the Inter-Tropical Convergence Zone before a new TC formation episode can occur. These results offer new insight into the processes that control the upper bound on the global number of TCs in the range of 80-100 annually.