I’m not sure I understand quite what you mean – the more people there are the more carbon dioxide there would be (as we breathe it out). Plants take it in so the less plants there are the less carbon dioxide gets re-absorbed (why cutting the rainforest down is a bad idea – its a carbon sink!).
Hi embmay! As Amy and Julia have said, plants thrive on CO2 and release oxygen. So yes less plants means less oxygen for us. But more CO2 from us means happier plants! (to a point).
Also, interesting fact – most of the oxygen on Earth actually doesn’t come from plants but instead from little photosynthesizing bacteria called cyanobacteria. They live everywhere, but a lot of them are in the oceans – this is one reason healthy oceans are important for us to have a healthy planet. In fact, the only reason we have any oxygen at all, enabling us to appear as oxygen breathing animals, is because of little microbes like modern cyanobacteria. The Earth had little oxygen until about 2.5 billion years ago when these guys invented photosynthesis! (Or I should say till photosynthesis evolved). That enabled more complex life to come a long like us.
The atmosphere of the Earth is really dependent on the life it harbors, its a delicate balance. So we don’t want to throw it too far off! (Although Earth will recover, we may not!).