Is there always the same amount of nitrogen and carbon in the world?

Yes, the total amount of nitrogen and carbon in this world (on planet Earth) is fixed. However, this amount may be in various chemical forms and/or in different spheres of our planet. 

The global biogeochemical cycles move these nutrients around. For example, nitrogen could be in the form of nitrogen gas (in our atmosphere), as ammonia, nitrite or nitrate in soil, our crops and in the bodies of animals. The amount of nitrogen may be temporarily fixed in one of these repositories (such as crops or animal bodies) and released when they decompose.

Similarly, carbon could be in the from of carbon dioxide, glucose (and as countless other organic molecules), carbonates, etc. Carbon compounds are found all around us, including in plants, animals, food, rocks, etc. 

Hence, the fixed amount of these nutrients cycles through various components of our Earth's global nutrient cycles.

Note that we are assuming that the bombardment of Earth by rocks (or meteorites) from space does not add any significant amount of nutrients to Earth.

Hope this helps.