Does Rain Forests of Amazon Really contributes to 20% of Earth's Oxygen ?

No, Amazon rain forests does not contribute to the 20% of earth's oxygen. In the scenario prior to the fire caused due to human acts, the complete forest was under steady state. Asuasual o2 was getting generated through photosynthesis and consumed through decay process.Suppose if the above mentioned two processes were deviating out of balance condition then there should have a change in the mass of wood in Amazon.

So for suppose if the complete Amazon rain forests were to disappear at once because of complete utilization of wood from forest to build some infrastructure then the levels of both oxygen and carbon dioxide would continue on the same level. Until, that is, the wood rots. Then the carbon dioxide levels would increase.

Besides for the biomass decrease from human-caused fires, the biomass of the Amazon rainforest has almost not at all been changing. This shows that there is no reduction in net carbon-dioxide from the atmosphere, so as no net oxygen is being released from carbon dioxide.

Recently it is noted that the Amazon biomass has been changing due to fires. Due to this, the wood and other carbohydrates in the trees are getting combined with atmospheric oxygen and producing CO2 and water. Thus the burning of the rainforests for sure contributes to global warming.

But under normal situations, when the biomass of the Amazon rainforest  is not varying, there won't be any net production of oxygen or carbon dioxide.

Many people mistakenly think that the Amazon produces net oxygen, double their error by using a backwards metaphor. They generally refer Amazon basin as the “lungs of the world.” But lungs are those that normally removes oxygen from the air and replaces it with carbon dioxide, but not the other way around.

So the 20% figure buzzing around is having some confusion got embedded in it. As per my view people might have calculated that 20% of the photosynthesis of the world takes place in the Amazon basin. But so does 20% of the consumption which may not be true.