The past average for SSN and SSBN parked at Longpo hovered around 4-5. This was in line with the overall fleet size and distribution (ie some were at Jianggezhuang). As the fleet size increases, so does the number of visible boats. Most submarines, regardless of propulsion, are parked "in the open", and as PLAN grows, I think these shelters will in fact decrease in importance.
During the Cold War and immediately afterwards American bases such as Norfolk routinely had ten or more SSN in berth at the same time. So this is quite normal.