Measuring on Google Maps, the in-between spacings of the submarine mooring piers is about 180 meters.
Given that submarines can be moored side-by-side by a pier, and assuming that each submarine needs about 20 meters of mooring width - That would mean 9 submarines can be moored abreast between each of the two submarine mooring piers.
But of course, that would be unrealistic. So having 2-3 submarines moored side-by-side on any one side of the submarine mooring piers there should do.
Therefore, theoretically speaking - The submarine mooring piers at Sanya could have a maximum mooring capacity of 24-36 submarines. And if the submarines can be moored in tandem, the theoretical maximum number could even be doubled to 48-72 submarines.
Of course, these are only hypothetical assumptions at best.