Most of the pictures if not all that you see in National Geographic are made by freelance photographers.
Magazines do not employ photographers full time anymore, if National Geographic does, there might be just few and the chances you get this job is pretty much none.
If you are talented and hardworking nothing is impossible but this is how it works.
Most of the work in the magazine are done by photographers that travel to remote places on their own, a lot of photographers work for government agencies , or universities research, or some other institution and this is how they make money , if there are very good photographers they can submit photos to National Geographic or any other magazine and maybe, maybe will be published.
So no one can tell you how much will you make.
You will never live in poverty if you live in USA, Australi, Europe and are hardworking person and if you graduate from the University , you will not live in poverty unless you start taking drugs , gamble or start drinking.
Actually most of the pictures in National Geographic are taken in places where poverty is very high so chances are that you will live among people and in places that are poor before you can take those great photographs. How will you deal with this? If you do not like poor places that I advice you to become a fashion photographer or commercial or hollywood. This are the kinds of photography that makes the most money and you will have the fancy things.
Great part of photographing for National Geographic is traveling to remote places and spend long time in very basic living conditions if you do not like it , that might be one reason this is not for you.
You have to learn languages.
You have to be a traveler.
I am sure everything is possible and if you feel really feel you would like to do it no matter what they you can give it a go and maybe some day will have a photo published in NAtional Geographic just understand that this is a magazine that receives probably 1000 or moreI would guess 5000 images to review on a monthly bases so your competition is very high.
Again I do not want to sound discouraging. Just do it, start photographing, go to school, read, practice, submit your work to National Geographic photo competitions.
Good luck.