9% of the Earth’s oxygen is produced by the Amazon rainforest.
In fact, under normal conditions (prior to human-caused fires) the Amazon forest is in a steady state.
Under normal situations, when the biomass of the Amazon is not changing, there is no net production of oxygen or carbon dioxide.
Where did the 20% figure come from? I’m guessing that ecologists have calculated that 20% of the photosynthesis of the world takes place in the Amazon basin. But so does 20% of the consumption.
Image credit: Amazon Forest/Unknown