Sir the solubility of oxygen depends on temperature. So if you see in the graph then you will find that the value 14 is near 0-degree celsius temperature and as temperature increases, DO value decreases. And DO saturated at 20 degrees celcius is 9.2 ppm and it is standard.
The maximum value of DO in water depends on the method you are introducing the Air/Oxygen to the liquid phase. Using a technology of nanobubbles the DO can reach up to 50 mg/l. This is the experimental result our team get from the application of NB application with a circulation of a given amount of water for 30 minutes. Therefore, the 14 mg/l is for a water in equilibrium with Air at STP in pparticular elevation.