How many garadosses do you need to get one red garadosse on Pokemon diamond?

Well to get a Red Gyarados its a random event, a red Gyarados is a shiny Pokemon its very rare almost as rare as any Legendary Pokemon you can find in game. The chances of encountering a shiny for any species is 1/8200 or something so if you encounter over 8000 Gyarados only 1 if any of those Gyarados will be shiny... or red. Well I hope that helps, keep trying maybe youll get lucky.