What is the maximum possible item discovery level in Dark Souls III, with 99 luck, Symbol of Avarice, etc. In short, every possible thing you can do to increase item discovery and what is the maximum number?
According to the Wiki:
The highest possible Item Discovery is 599, when equipped with two Crystal Sage's Rapiers, the Symbol of Avarice, the Gold Serpent Ring +2, using a Rusted Gold Coin and having 99 Luck.
As pointed out in the comments, if you have the Ringed City DLC, you can get a Gold Serpent Ring +3, raising the max by 15 to 614.