Does the Amazon produce 20% of the world's oxygen?

No. The Amazon produces a lot of oxygen through photosynthesis, but its plants, animals and microbes consume nearly all of it through respiration and decay, so the net contribution to the atmosphere is close to zero. Atmospheric oxygen is a 20% stockpile that changes very slowly. Roughly half of the oxygen produced on Earth each year comes from marine phytoplankton.

Source: Malhi et al. (2008), Science; Field et al. (1998), Science