In this paper, the electron density in radio frequency (RF) hollow cathode discharge (HCD) is investigated in experiments and in simulation based on two-dimensional (2D) fluid model. The role of hole diameter on plasma density in RF HCD have been explored at various gas pressures. It is found that the optimal hole size for maximum plasma density decreases along with the increase of gas pressure, which is confirmed by simulated results. The simulations reveal that the high plasma density is owing to the hollow cathode effect in the hollow cathode. It is obtained that the optimal hole diameter in RF HCD is approximately sum of twice thickness of plasma sheath and triple the electron-neutral mean free path.